Light-emitting device

ABSTRACT

An Object of the Present Invention is to provide a new light-emitting device with the use of an amorphous oxide. The light-emitting device has a light-emitting layer existing between first and second electrodes and a field effect transistor, of which the active layer is an amorphous.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.11/269,768, filed on Nov. 9, 2005, which claims the benefit of JapanesePatent Application No. 2004-326684, filed on Nov. 10, 2004. The contentsof the aforementioned applications are incorporated herein by referencein their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device using an oxide,and particularly using an organic EL element and an inorganic ELelement. The light-emitting device according to the present inventionalso relates to a top emission type or a bottom emission type.

2. Related Background Art

In recent years, a flat panel display (FPD) is in practical use as aresult of the progress of technologies relating to a liquid crystal andelectroluminescence (EL). The FPD is driven by an active matrix circuitconsisting of a field effect thin film transistor (TFT) using anamorphous silicon thin film or a polycrystalline silicon thin filmprovided on a glass substrate as an active layer.

On the other hand, an attempt has been made to use a lightweight andflexible resin substrate in place of the glass substrate, in quest offurther improvement in thinning, slimming and breakage resistance of anFPD. However, because a thermal process in a comparatively hightemperature is necessary to manufacture a transistor with the use of theabove described silicon thin film, it is difficult to directly form thesilicon thin film on a resin substrate with low heat resistance.

For this reason, a TFT using an oxide semiconductor thin film mainlycontaining ZnO for instance, which can be film-formed at a lowtemperature, has been actively developed (Japanese Patent ApplicationLaid-Open No. 2003-298062).

However, the TFT using a conventional oxide-semiconductor thin film doesnot reach a technical level of developing the applied technology,probably because of not having attained sufficient characteristics suchas in a TFT using silicon.

SUMMARY OF THE INVENTION

An object of the present invention is to provide the new light-emittingdevice, an electronograph and a display unit, which employ a transistorusing an oxide as an active layer.

According to an aspect of the present invention, there is provided alight-emitting device having a light-emitting element comprising firstand second electrodes and a light-emitting layer existing between thefirst and second electrodes, and a field effect transistor for drivingthe light-emitting element, wherein

the active layer of the field effect transistor is formed of anamorphous oxide having an electronic carrier concentration of less than10¹⁸/cm³.

The amorphous oxide preferably includes at least one of In, Zn and Sn.

Alternatively, the amorphous oxide is preferably any one selected fromthe group consisting of an oxide containing In, Zn and Sn; an oxidecontaining In and Zn; an oxide containing In and Sn; and an oxidecontaining In.

Alternatively, the amorphous oxide preferably includes In, Zn and Ga.

The light-emitting element and the field effect transistor arepreferably arranged on an optically transparent substrate, and a lightemitted from the light-emitting layer is output through the substrate.The field effect transistor is preferably arranged between the substrateand the light-emitting layer.

Alternatively, the light-emitting element and the field effecttransistor are preferably arranged on an optically transparentsubstrate, and a light emitted from the light-emitting layer is outputthrough the substrate and the amorphous oxide. The field effecttransistor is preferably arranged between the substrate and thelight-emitting layer.

In the light-emitting device, at least one of the drain electrode of thefield effect transistor and the second electrode is preferably formed ofan optically transparent electroconductive oxide.

The light-emitting element is preferably an electroluminescent element.

In the light-emitting device, a plurality of the light-emitting elementsare preferably arranged at least in a single row. The light-emittingelement is preferably arranged so as to be adjacent to the field effecttransistor.

According to another aspect of the present invention, there is providedan electrophotographic device having

a photoreceptor,

an electrifier for electrifying the photoreceptor,

an exposing light source for exposing the photoreceptor in order to forma latent image on the photoreceptor, and

a developing unit for developing the latent image, wherein

the exposing light source has the light-emitting device.

According to a still another aspect of the present invention, there isprovided a light-emitting device having a light-emitting elementcomprising first and second electrodes and a light-emitting layerexisting between the first and second electrodes, and a field effecttransistor for driving the light-emitting element, wherein

wherein an electron mobility of an active layer of the field effecttransistor tends to increase with increasing electron carrierconcentration.

According to a further aspect of the present invention, a light-emittingdevice having a light-emitting element comprising first and secondelectrodes and a light-emitting layer existing between the first andsecond electrodes, and a field effect transistor for driving thelight-emitting element, wherein

the active layer of the field effect transistor includes such atransparent amorphous-oxide semiconductor as to be capable of realizinga normally off state. The transparent amorphous-oxide semiconductorpreferably has an electronic carrier concentration of less than10¹⁸/cm³, which is sufficiently few in realizing the normally off state.

According to a further aspect of the present invention, an active matrixdisplay device comprising a light-emitting element comprising first andsecond electrodes and a light-emitting layer existing between the firstand second electrodes and a field effect transistor for driving thelight-emitting element, and a picture element circuit arranged into atwo-dimensional matrix form, wherein

the active layer of the field effect transistor includes such atransparent amorphous-oxide semiconductor as to be capable of realizinga normally off state. The transparent amorphous-oxide semiconductorpreferably has an electronic carrier concentration of less than10¹⁸/cm³, which is sufficiently few in realizing the normally off state.

According to a further aspect of the present invention, there isprovided a display article comprising;

a light-emitting element comprising first and second electrodes and alight-emitting layer existing between the first and second electrodesand a field effect transistor for driving the light-emitting element,wherein

an active layer of the field effect transistor includes an amorphousoxide semiconductor.

The amorphous oxide is preferably any one selected from the groupconsisting of an oxide containing In, Zn and Sn; an oxide containing Inand Zn; an oxide containing In and Sn; and an oxide containing In.

Alternatively, the transistor is preferably a normally-off typetransistor.

The present invention can provide a new light-emitting device,electronograph and active matrix display unit.

As a result of having studied a ZnO oxide semiconductor, the presentinventors found that a general process can not form a stable amorphousphase. Furthermore, it seemed that most of generally produced ZnOpresents a polycrystal phase, scatters carriers at interfaces betweenpolycrystal grains, and consequently cannot increase electron mobility.In addition, ZnO tends to form oxygen defects, consequently producesmany carrier electrons, and has difficulty in decreasing electricconductivity. For this reason, it was found that a TFT using the ZnOsemiconductor passes a large current between a source terminal and adrain terminal even when a gate voltage of a transistor is not applied,and can not realize a normally off operation. It also seems that the TFTusing the ZnO semiconductor has difficulty in increasing an ON/OFFratio.

Furthermore, the present inventors have studied on an amorphous oxidefilm Zn_(x)M_(y)In_(z)O_((x+3y/2+3z/2)) (wherein M is at least oneelement of Al and Ga), described in Japanese Patent ApplicationLaid-Open No. 2000-044236. The material has an electronic carrierconcentration of 1×10¹⁸/cm³ or more, and accordingly is suitable forbeing used simply as a transparent electrode. However, it was found thatthe material is not suitable for a normally off type of a TFT, becausewhen the TFT employs an oxide with the electronic carrier concentrationof 1×10¹⁸/cm³ or more for its channel layer, the TFT can not provide asufficient ON/OFF ratio.

In other words, a conventional amorphous oxide film could not obtain anelectronic carrier concentration of less than 1×10¹⁸/cm³.

The present inventors have energetically researched the properties ofInGaO₃(ZnO)_(m) and a film-forming condition for the material, and as aresult, have found that electronic carrier concentration can be reducedto less than 1×10¹⁸/cm³ by controlling the condition of oxygenatmosphere during forming the film.

Consequently, the present inventors found that a TFT prepared by usingan amorphous oxide containing electronic carriers with a concentrationof less than 1×10¹⁸/cm³ as an active layer of a field effect transistorcan show desired characteristics, and can be applied to a flat paneldisplay such as a light-emitting device.

As for an electronograph provided with a linearly arrayed light sourceand a photoreceptor using a light-emitting device according to thepresent invention, there are a copying machine, a page printer and anintegral drum cartridge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic drawing showing a relationship betweenelectronic carrier concentration in an In—Ga—Zn—O-based amorphous filmformed with a pulsed laser vapor deposition method and a oxygen partialpressure while the film is being formed;

FIG. 2 is a diagrammatic drawing showing a relationship between electricconductivity in an In—Ga—Zn—O-based amorphous film formed with asputtering method using an argon gas and a oxygen partial pressure whilethe film is being formed;

FIG. 3 is a diagrammatic drawing showing a relationship between thenumber of electronic carriers and electron mobility in anIn—Ga—Zn—O-based amorphous film formed with a pulsed laser vapordeposition method;

FIGS. 4A, 4B and 4C are diagrammatic drawings showing the changes ofelectric conductivity, carrier concentration and electron mobility withrespect to a value of x of InGaO₃(Zn_(1-x)Mg_(x)O) film-formed with apulsed laser vapor deposition method in an atmosphere having an oxygenpartial pressure of 0.8 Pa;

FIG. 5 is a schematic block diagram showing the structure of a top gatetype MISFET element prepared in Embodiment 1;

FIG. 6 is a diagrammatic drawing showing current-voltage characteristicsof a top gate type MISFET element prepared in Embodiment 1;

FIG. 7 is a schematic block diagram showing a cross section of alight-emitting device according to the present invention;

FIG. 8 is a circuit diagram when a light-emitting device according tothe present invention is used for a display;

FIG. 9 is a diagrammatic drawing showing a cross section of alight-emitting device according to the present invention;

FIG. 10 is a diagrammatic drawing describing the electrical connectionof a linearly arrayed light source according to the present invention;

FIG. 11 is a sectional view describing a configuration example of alinearly arrayed light source according to the present invention;

FIG. 12 is a sectional view (right above arrangement) describing aconfiguration example of a linearly arrayed light source according tothe present invention;

FIG. 13 is a diagrammatic drawing showing an arrangement example of acopying machine, a page printer, a photoconductor drum in an integraldrum cartridge and a linearly arrayed light source;

FIG. 14 is a schematic block diagram showing a pulsed laser vapordeposition apparatus; and

FIG. 15 is a schematic block diagram showing a sputtering film-formingapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A basic configuration according to the present invention will be nowdescribed with reference to FIG. 7 which shows the first embodiment.

In the drawing, numerical character 70 denotes a drain electrode,numerical character 71 a substrate, numerical character 72 an activelayer, numerical character 73 a gate insulation film, numericalcharacter 74 a gate electrode, numerical character 75 a sourceelectrode, numerical character 77 a second electrode, numericalcharacter 78 a light-emitting layer and numerical character 79 a firstelectrode. In the first embodimentembodiment, the second electrode(hereafter called a “bottom electrode”) is arranged nearer to asubstrate than a light-emitting layer is, the light-emitting layer is anorganic EL layer, and the first electrode (hereafter called a “counterelectrode”) is arranged more distant from the substrate than thelight-emitting layer is. Gaps between the above described components arefilled with an interlayer insulation layer 76.

At first, each component will be described in detail.

(1. Substrate)

Glass is generally used for a material of a substrate in alight-emitting device. However, because a TFT used in the presentinvention can be formed at a low temperature, a plastic substrate whichhas been difficult to be used in an active matrix can be used in thepresent invention. Thereby, the light-emitting device can be providedwhich is lightweight, is hardly damaged and bendable to some extent. Asa matter of course, a semiconductor substrate such as Si and a ceramicsubstrate can be used. A substrate having an insulation layer providedon a metallic base plate can also be used as long as the substrate isflat.

(2. Transistor)

An active layer of a transistor may be any material as long as it hasdesired characteristics, specifically, an electronic carrierconcentration of less than 1×10¹⁸/cm³ and an electron mobility of morethan 1 cm²/(V-second). The material includes, for instance, anIn—Ga—Zn—O-based amorphous-oxide semiconductor. The amorphous oxide is atransparent film. Here, the word, transparence, not only means the caseof being substantially transparent for visible light, but also includesthe case of being optically transparent for at least one part of lightin a visible range. As for the optical transparency, theIn—Ga—Zn—O-based amorphous-oxide semiconductor preferably has atransmittance of 50% or higher, and more preferably of 80% or higher.The above composition can contain substituted or added Mg. When alight-emitting device employs a TFT using the In—Ga—Zn—O-based activelayer, it shows a useful performance, because the TFT supplies asufficient driving force of both voltage and current to an organic ELelement.

The amorphous oxide will be separately described in detail.

A sputtering technique and a pulsed laser vapor deposition method aresuitable for forming an active layer as was described above, but suchvarious sputtering techniques as to have an advantage in productivityare preferable. It is also effective to appropriately interpose a bufferlayer between the active layer and the substrate.

As a gate insulation film, any one of Al₂O₃, Y₂O₃ and HfO₂, or a mixedcrystal compound containing at least two or more compounds thereof ispreferable, but may be another compound.

A usable source electrode and drain electrode include anelectroconductive oxide represented by ITO and a metal such as Au.However, the electrode preferably can be ohmically or nearly ohmicallyconnected with an active layer. The drain electrode may also be directlyconnected with a light-emitting layer without bypassing a secondelectrode.

(3. Light-Emitting Layer)

A light-emitting layer is not limited as long as it can be driven by aTFT, but an organic EL is particularly advantageous. The organic ELlayer 78 used in the present invention is rarely used as a single layer,but is often used in a configuration consisting of a plurality oflayers, as is shown in the following. In the following, an “electrontransport layer” means a light-emitting layer having anelectron-transporting function.

hole transport layer/light-emitting layer+electron transport layer

hole transport layer/light-emitting layer/electron transport layer

hole injection layer/hole transport layer/light-emitting layer/electrontransport layer

hole injection layer/hole transport layer/light-emitting layer/electrontransport layer/electron injection layer

An electron barrier layer and an adhesion improvement layer areoccasionally interposed between a plurality of the layers.

In general, there are two principles of fluorescence and phosphorescencein a light-emitting principle of a light-emitting layer, but thephosphorescence is effective from a viewpoint of luminous efficiency. Aniridium complex is useful as a phosphorescent material. Both of a lowmolecule polymer and a high molecule polymer can be used as a polymerused in a base metal for the light-emitting layer. When the low moleculepolymer is employed, the light-emitting layer can be generally formedwith a vapor deposition technique, and when the high molecule polymer isemployed, the light-emitting layer with an ink jet method or a printingprocess can be formed. For example, the low MW (Molecular Weight)materials include an amine complex, anthracene, a rare earth complex anda noble metal complex; and the high MW materials includes a n conjugatepolymer and a pigment-containing polymer.

An electron injection layer includes an alkali metal, an alkaline earthmetal, a compound thereof, and an organic layer doped with an alkalimetal. In addition, an electron transport layer includes an aluminumcomplex, oxadiazole, triazole and phenanthroline.

A hole injection layer includes arylamines, phthalocyanines and anorganic layer doped with a Lewis acid; and a hole transport layerincludes arylamine.

By the way, FIG. 7 shows a configuration example of an organic ELelement, but the same configuration can be used for an inorganic ELelement.

(4. First Electrode)

A first electrode will be described for the case of being a counterelectrode. A preferred material for the counter electrode differsdepending on whether it is used in a top emission type or a bottomemission type, and is used as a cathode or an anode.

When a counter electrode is used in a top emission type, transparency isrequired; and when it is used as an anode, a usable material includesITO, electroconductive tintintin oxide, electroconductive ZnO andIn—Zn—O and In—Ga—Zn—O-based oxide having an electronic carrierconcentration of 1×10¹⁸/cm³ or more, which are all transparentelectroconductive oxides. When it is used as a cathode, the counterelectrode can be formed by forming an alloy doped with an alkali metalor an alkaline earth metal into a film with several tens of nanometersor thinner and forming the transparent electroconductive oxide on theupper part.

When it is used in a bottom emission type, transparency is not needed,so that when it is used as an anode, an Au alloy and a Pt alloy areusable for it, and when it is used as a cathode, Ag-added Mg, Li-addedAl, a silicide, a boride and a nitride can be used.

(5. Second Electrode)

A second electrode is connected to a drain electrode. The secondelectrode may have the composition as or a different composition fromthat of the drain electrode.

The second electrode may be a bottom electrode. The bottom electrode maybe formed in a layer form along a substrate or a light-emitting layer.

When a light-emitting layer is a current injection type as isrepresented by an organic EL element, there is a preferred bottomelectrode according to the configuration.

When a light-emitting layer connected to a bottom electrode is acathode, the bottom electrode preferably is a metal having a small workfunction. The bottom electrode includes, for instance, Ag-added Mg,Li-added Al, a silicide, a boride and a nitride. In this case, it ismore advantageous to be connected with a drain part of a TFT throughwiring than to be directly connected with it.

When a light-emitting layer connected to a bottom electrode is an anode,the bottom electrode preferably is a metal having a large work function.The bottom electrode includes, for instance, ITO, electroconductivetintin oxide, electroconductive ZnO, In—Zn—O, a Pt alloy and an Aualloy. In addition, In—Ga—Zn—O-based oxide with an electronic carrierconcentration of 1×10¹⁸/cm³ or higher can be used. In this case, thehigher is the carrier concentration, the more preferable as the bottomelectrode is the oxide, which is different from the case used for a TFT.For instance, the carrier concentration of 1×10¹⁹/cm³ or more ispreferable. When the bottom electrode is made of ITO or In—Ga—Zn—O basedoxide (with high carrier concentration), it can provide a high open-arearatio even if being employed in a bottom emission type, because of beingtransparent. When the bottom electrode is directly connected with adrain electrode, particularly ITO, the above described In—Ga—Zn—O basedoxide (with high carrier concentration) and an Au alloy are particularlypreferable for it.

When a bottom electrode is directly connected with a drain electrode,the bottom electrode is preferably a hole injection type. Particularlypreferable materials for the bottom electrode are ITO, ZnO doped with Alor Ga, and In—Ga—Zn—O-based oxide with a carrier concentration of1×10¹⁸/cm³ or more. Particularly, when the In—Ga—Zn—O-based oxide isemployed as an electrode and an active layer, the carrier concentrationof a part of an active layer In—Ga—Zn—O can be increased by a techniqueof introducing oxygen defect therein, or the like, the light-emittingdevice is simply and effectively configured. In this case, at a glance,a hole transport layer and a hole injection layer are formed on theactive layer. This configuration shall be in a range of the presentinvention. Specifically, it means the configuration in which the bottomelectrode and the drain electrode are integrated with one part of theactive layer.

(6. Interlayer Insulation Layer)

Particularly, when a second electrode is a bottom electrode, aninterlayer insulation layer 76 considered to be an underlayer of thebottom electrode 77 can employ the very same material as in a gateinsulation film. As a matter of course, the insulation layer with theuse of the other material can be formed in order to obtain a flat layer.For instance, a polyimide film can be spin-coated, and silicon oxide canbe formed with a plasma CVD method, a PECVD method and a LPCVD method orby coating and baking siliconalkoxide. The interlayer insulation layerneeds to appropriately have a contact hole for connection to a sourceelectrode or a drain electrode formed therein.

(7. Electrode Wire and Others)

An electrode wire such as a scan electrode wire and a signal electrodewire which are a gate electrode wire can employ a metal such as Al, Crand W and silicide such as WSi as the material.

In the next place, a relationship among each of components will bedescribed in detail.

At first, a first embodimentembodiment will be described with referenceto FIG. 7, in which a bottom electrode is partially connected with adrain electrode by wiring.

First Embodimentembodiment

A source electrode 75 and a drain electrode 70 are directly connectedwith an active layer 72, and the current passing through the activelayer 72 is controlled by a gate electrode 74 through a gate insulationfilm 73.

An organic EL layer 78 of a light-emitting layer is connected with adrain electrode 70, through a bottom electrode 77 and a wire in acontact hole. An interlayer insulation layer 76 exists between thebottom electrode 77 and a TFT section to electrically insulate them. Theinterlayer insulation layer 76 does not necessarily need to be a singlelayer, but generally consists of insulation layers arranged on the gateinsulation film and on the upper part of the gate electrode, and thegeneral interlayer insulation layer for a flattening purpose.

A counter electrode 79 exists on the upper part of the organic EL layer78, and applies voltage to the organic EL layer 78 to make it emit lightwhen a TFT is turned ON.

Here, a drain electrode 70 is electrically connected with a secondelectrode, or is the second electrode in itself.

FIG. 7 shows an example of a high open-area ratio in which an organic ELlayer 78 exists even on the top of a TFT, but the organic EL layer 78may be formed on other parts than a TFT part, as long as it does notcauses problem with application. However, when the organic EL layer 78is used in a configuration shown by FIG. 7, a lower part of the organicEL layer 78 is preferably as flat as possible.

Although drain electrode 70 is connected to bottom electrode 77 in FIG.7, source electrode 75 is connected to bottom electrode 77, depending ona manner of the use. That is, the present invention is characterized inconnecting any one of the source electrode and the drain electrode of atransistor containing an amorphous oxide to an electrode sandwiching alight-emitting layer. In case that bottom electrode 77 under thelight-emitting layer 78 is an anode, a constitution wherein a sourceelectrode of the TFT is connected to the anode is also preferable.

(About Preparation of Light-Emitting Device)

Here, a process for manufacturing a light-emitting device according tothe present invention will be described, with reference to an example ofthe configuration in which a drain electrode is connected with a bottomelectrode through wiring, and an organic EL is employed as alight-emitting element.

(Preparation of Transistor)

A transistor is prepared by the steps of: depositing a thin film of anIn—Ga—Zn—O-based amorphous-oxide semiconductor on a glass substrate intoa thickness of 120 nm with a pulsed laser vapor deposition method, insuch a condition as to make the electron carrier concentration into1×10¹⁸/cm³, which will be described in detail later, while using apolycrystal sintered compact having the composition of InGaO₃(ZnO)₄ as atarget;

further layering the film of InGaO₃(ZnO)₄ with high electroconductivitythereon into the thickness of 30 nm with the pulsed laser depositionmethod in a chamber having an oxygen partial pressure controlled to lessthan 1 Pa, and forming the film of Au thereon into the thickness of 50nm as a source electrode and a drain electrode with an electron beamvapor deposition method; and

further forming each film of Y₂O₃ as a gate insulation film and of Au asa gate electrode into the thickness of respectively 90 nm and 50 nm withthe electron beam vapor deposition method. In a series of the abovedescribed processes, each layer is formed into a desired size with aphotolithographic method and a lift-off technology. Furthermore, aninsulation layer is formed on them with a similar method. Then, acontact hole for a drain electrode is also formed therein.

(Preparation of Bottom Electrode Layer)

After that, a bottom electrode is formed by forming the film of ITO intothe thickness of 300 nm with a sputtering technique, and then isconnected with a drain electrode and the bottom electrode, throughwiring formed in a contact hole.

(Preparation of Organic EL Light-Emitting Layer)

In the next step, the organic EL light-emitting layer is prepared byforming the following films with a heat resistance vapor depositionmethod: the film of 4,4′-bis[N,N-diamino]-4″-phenyl-triphenyl amine withthe thickness of 60 nm as a hole injection layer; the film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl with the thickness of 20nm as a hole transport layer thereon; the film of4,4′-bis(2,2-diphenyl)vinyl with the thickness of 40 nm as alight-emitting layer; and the film of tris(8-quinolinol) aluminum withthe thickness of 20 nm as an electron transport layer.

(Preparation of Counter Electrode)

Finally, a counter electrode is prepared by forming the film of Al andAg alloy into the thickness of 50 nm with a dual vapor depositionmethod, and the film of Al into the thickness of 50 nm.

When the above described element is driven by contacting a probethereon, blue light is emitted from a back side of a substrate, in otherwords, a bottom emission type of the element is obtained.

In the present embodimentembodiment, it is important that theIn—Ga—Zn—O-based amorphous-oxide semiconductor acquires desiredelectronic carrier concentration by controlling an amount of oxygendefect.

In the above description, an amount of oxygen (an amount of oxygendefect) in a transparent oxide film is controlled by forming the film inan atmosphere including a predetermined concentration of oxygen, but itis also preferable to control (reduce or increase) the amount of oxygendefect, by post-treating the oxide film formed in the previous step inthe atmosphere including oxygen.

In order to effectively control an amount of oxygen defect, atemperature in an atmosphere including oxygen is controlled to 0° C. orhigher and 300° C. or lower, preferably to 25° C. or higher and 250° C.or lower, and further preferably to 100° C. or higher and 200° C. orlower.

As a matter of course, the film may be formed in an atmosphere includingoxygen and then be post-treated in the atmosphere including oxygen. Inaddition, the film may be formed in an atmosphere of which the oxygenpartial pressure is not controlled, and be post-treated in theatmosphere including oxygen, if the method provides predeterminedelectronic carrier concentration (less than 1×10¹⁸/cm³).

Here, a lower limit of electronic carrier concentration in the presentinvention depends on what kind of an element, a circuit and a device anobtained oxide film is used for, but for instance, is 1×10¹⁴/cm³ orhigher.

In the next place, with reference to FIG. 9, a secondembodimentembodiment will be described in which a bottom electrode isdirectly connected with a drain electrode without interposing wiringbetween them.

Second Embodiment

In FIG. 9, reference numeral 91 denotes a substrate, reference numeral92 an active layer made of a semiconductor material peculiar to thepresent invention, reference numeral 93 a gate insulation film,reference numeral 94 a gate electrode, reference numeral 95 a sourceelectrode, reference numeral 96 an insulation layer, and referencenumeral 97 a bottom electrode. The bottom electrode 97 is layered on adrain electrode or is identical to the drain electrode, in other words,is the drain electrode in itself. Reference numeral 98 denotes anorganic EL layer, and reference numeral 99 a counter electrode.

The present embodiment has basically the same type of configuration inthe first embodiment in which a part of a bottom electrode contacts witha drain electrode through wiring, but has a different configuration inwhich the drain electrode and a light-emitting layer are layered on thesame overlapping area when viewed from the position above a substrate91, and the bottom electrode exists between them.

A drain electrode may be the same member as a bottom electrode. In thiscase, the drain electrode needs to have a capability of effectivelyinjecting an electron or a hole into an organic EL layer.

It is preferable to control electronic carrier density in a part of anactive layer corresponding to a bottom part of a drain electrode to1×10¹⁸/cm³ or higher by increasing oxygen defects. Then, it can work asthe drain electrode and a bottom electrode at the same time. In thiscase, it is preferable that the active layer is made of an In—Ga—Zn—Ofilm, functions as a hole injection layer, and is connected with ananode part of a light-emitting layer.

FIG. 9 shows an example of a light-emitting layer arranged in an upperpart of a TFT layer, but a configuration is acceptable in which acounter electrode of a first electrode and a bottom electrode of asecond electrode are turned upside down, and the first electrode isdirectly layered on a drain electrode, if it does not cause a functionalproblem. In this case, the second electrode which has been the bottomelectrode exists seemingly in the upper part, but it is functionally thesame as long as it is connected with the drain electrode. Particularly,when an inorganic EL layer is employed, the inverse case can be adopted,because of having higher processing resistance than an organic EL layerhas.

In a preferred configuration of a light-emitting device according to thepresent invention, first and second electrodes are a counter electrodeand a bottom electrode, a field effect transistor is a TFT, an activelayer of the TFT includes In, Ga and Zn, at least one part of the activelayer is an amorphous oxide, and one part of a light-emitting layer iselectrically connected with a drain electrode of the TFT.

In a light-emitting device according to the present invention, one partof a light-emitting layer is directly connected to a drain electrode, orone part of the light-emitting layer may be connected with the drainelectrode through wiring. It is preferable that one part of thusconnected light-emitting layer is an anode side or a cathode side of thelight-emitting layer.

It is preferable that at least one of a drain electrode and the abovedescribed bottom electrode is a transparent electroconductive oxide.

In the next place, a third embodiment which is a configuration exampleof the application to a display will be described with reference to FIG.8.

Although drain electrode 97 is utilized as the bottom electrode in FIG.9, source electrode 95 may be utilized as the bottom electrode,depending on a constitution of layer structure of the light-emittinglayer. That is, the present invention is characterized in connecting andso forth any one of the source electrode and the drain electrode of atransistor containing an amorphous oxide to an electrode sandwiching alight-emitting layer.

Third Embodiment

In FIG. 8, reference numeral 81 denotes a transistor for driving anorganic EL layer 84, which passes a current to a light-emitting elementthat has the organic EL layer 84 and a pair of electrodes sandwichingthe organic EL layer 84. Reference numeral 82 is a transistor 2 forselecting a picture element, which supplies a picture signal fordetermining a current to be passed to the light-emitting element, to agate of the transistor 81.

In addition, a condenser 83 is placed in order to hold a selectedcondition, stores electrical charges between a common electrode wire 87and a source electrode part of the transistor 2, and retains the signalof the gate of the transistor 1. The picture element is selected anddetermined by a scan electrode wire 85 and a signal electrode wire 86.

The configuration will be now more specifically described.

At the same time when a row selection signal is applied to a gateelectrode from a driver circuit (not shown) through a scan electrode 85in a form of a pulse signal, the picture signal is applied to atransistor 82 from another driver circuit (not shown) through a signalelectrode 86 to select a picture element.

At this time, the transistor 82 is tuned ON and an electrical charge isstored in a condenser 83 placed between a signal electrode wire 86 andthe source electrode of the transistor 82. Thereby, a gate voltage ofthe transistor 81 is held in a desired voltage corresponding to thepicture signal, and the transistor 81 passes a current corresponding tothe picture signal between the source electrode and a drain electrode.The state is retained till the transistors receive the next signal.While the transistor 81 passes the current, the current is continuouslysupplied to an organic EL layer 84 as well, and light emission ismaintained.

The example of the FIG. 8 shows a configuration using two transistorsand one condenser for one picture element, but the configuration mayinclude more transistors in order to improve the performance.

It is essential that a light-emitting device can realize a normally offtype of a TFT, by using a TFT having an active layer of an amorphousoxide illustrated by In—Ga—Zn—O-based oxide which is transparent and canbe formed at a low temperature according to the present invention, in atransistor part, and consequently inhibits unnecessary light emission.The above TFT can also provide an electronograph and a display unit withhigh contrast.

Fourth Embodiment

A fourth embodiment according to the present invention is a bottomemission type of a light-emitting device as shown in FIGS. 11 and 12.

Specifically, the light-emitting device has the above describedlight-emitting layer and the above described field effect transistorarranged on an optically transparent substrate, and emits light from theabove described light-emitting layer through the above describedsubstrate. In FIG. 11, an active layer (a channel layer 2101) formed ofan amorphous oxide arranged also right under a light-emitting element2160, but needs not to be arranged right under it. As a matter ofcourse, in a configuration of FIG. 11, light emitted from thelight-emitting layer 2108 passes through the active layer formed of theabove described amorphous oxide and the substrate 2100.

By the way, a light source can be produced by disposing a light-emittingdevice according to the present invention in one dimension, and can makean apparatus by combining itself with a photoconductor drum 2350 of aphotoreceptor as shown in FIG. 13. Specifically, an electronograph canbe composed from the photoreceptor 2350, an electrifier (not shown) forelectrifying the photoreceptor, an exposing light source 2300 forilluminating the photoreceptor to light 2301 in order to form a latentimage on the photoreceptor and a developing unit (not shown) fordeveloping the above described latent image.

An amorphous oxide described in the above described first and secondembodiments will be described in detail below.

(Amorphous Oxide)

The active layer employed in the above Embodiments 1 to 3 of theinvention is explained below.

The electron carrier concentration in the amorphous oxide in the presentinvention is a value measured at a room temperature. The roomtemperature is a temperature in the range from 0° C. to about 40° C.,for example, 25° C. The electron carrier concentration in the amorphousoxide in the present invention need not be less than 10¹⁸/cm³ throughoutthe entire range from 0 to 40° C. For example, the electron carrierconcentration of less than 10¹⁸/cm³ at a temperature of 25° C. isacceptable. At a lower electron carrier concentration, not more than1×10¹⁷/cm³, or not more than 1×10¹⁶/cm³, a normally-off TFT can beproduced at a high yield.

In the present specification, the description “less than 10¹⁸/cm³” means“preferably less than 1×10¹⁸/cm³ and more preferably less than1.0×10¹⁸/cm³”. The electron carrier concentration can be measured bymeasurement of a Hall Effect.

The amorphous oxide in the present invention is an oxide which exhibitsa halo pattern and no characteristic diffraction line in an X-raydiffraction spectrometry.

In the amorphous oxide of the present invention, the lower limit of theelectron carrier concentration is, for example, 1×10¹²/cm³, but is notlimited insofar as it is applicable as a channel layer of a TFT.

Accordingly, in the present invention, the electron carrierconcentration is adjusted by controlling the material, compositionratio, production conditions, and so forth of the amorphous oxide as inthe Examples described later to be in the range, for instance, from1×10¹²/cm³ to 1×10¹⁸/cm³, preferably from 1×10¹³/cm³ to 1×10¹⁷/cm³, morepreferably from 1×10¹⁵/cm³ to 1×10¹⁶/cm³.

The amorphous oxide, other than the InZnGa oxides, can be selectedsuitably from In oxides, In_(x)Zn_(1-x) oxides (0.2≦x≦1), In_(x)Sn_(1-x)oxides (0.8≦x≦1), and In_(x)(Zn, Sn)_(1-x) oxides (0.15≦x≦1). TheIn_(x)(Zn, Sn)_(1-x) oxide can also be represented byIn_(x)(Zn_(y)Sn_(1-y))_(1-x) (0≦y≦1).

When the In oxide contains neither Zn nor Sn, the In can be partlysubstituted by Ga: In_(x)Ga_(1-x) oxide (0≦x≦1).

An amorphous oxide of an electron carrier concentration of 1×10¹⁸/cm³which is prepared by the inventors of the present invention is describedbelow in detail.

One group of the aforementioned oxides are characteristicallyconstituted of In—Ga—Zn—O, represented by InGaO₃(ZnO)_(m) (m: a naturalnumber of less than 6) in a crystal state, and containing electroncarriers at a concentration of less than 1×10¹⁸/cm³.

The other group of the aforementioned oxides are characteristicallyconstituted of In—Ga—Zn—Mg—O, represented by InGaO₃(Zn_(1-x)Mg_(x)O)_(m)(m: a natural number of less than 6, and 0≦x≦1) in a crystal state, andcontaining electron carriers at a concentration of less than 1×10¹⁸/cm³.

The film constituted of such an oxide is preferably designed to exhibitpreferably an electron mobility of higher than 1 cm²/V·sec.

By use of the above film as the channel layer, a TFT can be preparedwhich is normally-off with a gate current of less than 0.1 microamperein a transistor off-state, having an on-off ratio of higher than 1×10³,being transparent to visible light and flexible.

In the above film, the electron mobility increases with the increase ofthe conduction electrons. The substrate for forming the transparent filmincludes glass plates, plastic plates, and plastic films.

In using the above amorphous oxide film as the channel layer, at leastone of layers constituted of Al₂O₃, Y₂O₃ or HfO₂, or a mixed crystalcompound thereof is useful as the gate insulator.

In a preferred embodiment, the film is formed in an oxygengas-containing atmosphere without intentional addition of an impurityfor increasing the electric resistance to the amorphous oxide.

The inventors of the present invention found that the amorphous thinfilms of semi-insulating oxides have characteristics that the electronmobility therein increases with increase in number of conductionelectrons, and further found that a TFT prepared by use of the film isimproved in transistor characteristics such as the on-off ratio, thesaturation current in a pinch-off state, and the switching rate. Thus anormally-off type TFT can be produced by use of the amorphous oxide.

By use of the amorphous oxide thin film as the channel layer of a filmtransistor, the electron mobility can be made higher than 1 cm²/V·sec,preferably higher than 5 cm²/V·sec. The current between the drainterminal and the source terminal at an off-state (no gate voltageapplied) can be controlled to be less than 10 microamperes, preferablyless than more than 0.1 microamperes at the carrier concentration oflower than 1×10¹⁸/cm³, preferably lower than 1×10¹⁶/cm³. Further by useof this thin film, the saturation current after pinch-off can be raisedto 10 microamperes or more and the on-off ratio can be raised to behigher than 1×10³ for the electron mobility higher than 1 cm²/V·sec,preferably higher than 5 cm²/V·sec.

In a pinch-off state of the TFT, a high voltage is being applied to thegate terminal, and electrons are existing in a high density in thechannel. Therefore, according to the present invention, the saturationcurrent can be increased in correspondence with the increase of theelectron mobility. Thereby, the transistor characteristics can beimproved, such as increase of the on-off ratio, increase of thesaturation current, and increase of the switching rate. In contrast, ina usual compound, the increase of electrons decreases the electronmobility owing to collision between electrons.

The structure of the aforementioned TFT may be a stagger (top gate)structure in which a gate insulator and a gate terminal are successivelyformed on a semiconductor channel layer, or a reversed stagger (bottomgate) structure in which a gate insulator and a semiconductor channellayer successively on a gate terminal.

(First Process for Film Formation: PLD Process)

The amorphous oxide thin film having the composition InGaO₃(ZnO)_(m) (m:a natural number of less than 6) in a crystal state is stable up to ahigh temperature of 800° C. or higher when m is less than 6, whereaswith increase of m, namely with increase of the ratio of ZnO to InGaO₃near to the composition of ZnO, the oxide tends to crystallize.Therefore, the value m of the oxide is preferably less than 6 for use asthe channel layer of the amorphous TFT.

The film formation is conducted preferably by a gas phase film formationprocess by use of a target of a polycrystalline sintered compact havinga composition InGaO₃(ZnO)_(m). Of the gas phase film formationprocesses, sputtering, and pulse laser vapor deposition are suitable.The sputtering is particularly suitable for the mass-production.

However, in formation of the amorphous film under usual conditions,oxygen defect can occur, so that the electron carrier concentration ofless than 1×10¹⁸/cm³ and electric conductivity of less the 10 S/cmcannot be achieved. With such a film, a normally-off transistor cannotbe constituted.

The inventors of the present invention produced an In—Ga—Zn—O film by apulse laser vapor deposition by use of the apparatus shown in FIG. 14.

The film-forming was carried out by using such a PLD film-formingapparatus as shown in FIG. 14.

In FIG. 14, the numerals indicate the followings: 701, an RP (rotarypump); 702, a TMP (turbo molecular pump); 703, a preliminary chamber;704, an electron gun for RHEED; 705, a substrate-holding means forrotating and vertically moving the substrate; 706, a laser-introducingwindow; 707, a substrate; 708, a target; 709, a radical source; 710, agas inlet; 711, a target-holding means for rotating and verticallymoving the target; 712, a by-pass line; 713, a main line; 714, a TMP(turbo molecular pump); 715, an RP (rotary pump); 716, a titanium getterpump; 717, a shutter; 718, an IG (ion manometer); 719, a PG (Piranigage); 720, a BG (baratron gage); and 721, a growth chamber.

An In—Ga—Zn—O type amorphous oxide semiconductor thin film was depositedon an SiO₂ glass substrate (Corning Co.: 1737) by a pulse laser vapordeposition employing a KrF excimer laser. As the pretreatment before thedeposition, the substrate was washed ultrasonically for defatting withacetone, ethanol, and ultrapure water each for five minutes, and driedin the air at 100° C.

The polycrystalline target was an InGaO₃(ZnO)₄ sintered compact (size:20 mm diameter, 5 mm thick), which had been prepared by wet-mixingIn₂O₃, Ga₂O₃, and ZnO (each 4N reagent) as the source material (solvent:ethanol), calcining the mixture (1000° C., 2 hours), dry-crushing it,and sintering it (1550° C., 2 hours). The target had an electroconductivity of 90 S/cm.

The film formation was conducted by controlling the final vacuum of thegrowth chamber to be 2×10⁻⁶ Pa, and the oxygen partial pressure duringthe growth to be 6.5 Pa. The oxygen partial pressure in growth chamber721 was 6.5 Pa, and the substrate temperature was 25° C. The distancebetween target 708 and film-holding substrate 707 was 30 mm, the powerintroduced through introduction window 716 was in the range of 1.5-3mJ/cm²/pulse. The pulse width was 20 nsec, the repeating frequency was10 Hz, and the irradiation spot size was 1×1 mm square. Under the aboveconditions, the film was formed at a rate of 7 nm/min.

The resulting thin film was examined by small angle X-ray scatteringmethod (SAXS) (thin film method, incidence angle: 0.5°): no cleardiffraction peak was observed. Thus the obtained In—Ga—Zn—O type thinfilm was judged to be amorphous. From X-ray reflectivity and its patternanalysis, the mean square roughness (Rrms) was found to be about 0.5 nm,and the film thickness to be about 120 nm. From fluorescence X-rayspectrometric analysis (XRF), the metal composition of the film wasfound to be In:Ga:Zn=0.98:1.02:4. The electric conductivity was lessthan about 1×10⁻² S/cm. The electron carrier concentration was estimatedto be not more than 1×10⁻¹⁶/cm³. The electron mobility was estimated tobe about 5 cm²/V·sec. From light absorption spectrum analysis, theoptical band gap energy breadth of the resulting amorphous thin film wasestimated to be about 3 eV.

The above results show that the obtained In—Ga—Zn—O type thin film is atransparent flat thin film having an amorphous phase of a compositionnear to a crystalline InGaO₃(ZnO)₄, having less oxygen defect, andhaving lower electric conductivity.

The above film formation is explained specifically by reference toFIG. 1. FIG. 1 shows dependency of the electron carrier concentration inthe formed transparent amorphous oxide thin film on the oxygen partialpressure for the film of a composition of InGaO₃(ZnO)_(m) (m: an integerless than 6) in an assumed crystalline state under the same filmformation conditions as in the above Example.

By formation of the film in an atmosphere having an oxygen partialpressure of higher than 4.5 Pa under the same conditions as in the aboveExample, the electron carrier concentration could be lowered to lessthan 1×10¹⁸/cm³ as shown in FIG. 1. In this film formation, thesubstrate was kept nearly at room temperature without intentionalheating. For use of a flexible plastic film as the substrate, thesubstrate temperature is kept preferably at a temperature lower than100° C.

The higher oxygen partial pressure enables decrease of the electroncarrier concentration. For instance, as shown in FIG. 1, the thinInGaO₃(ZnO)₄ film formed at the substrate temperature of 25° C. and theoxygen partial pressure of 5 Pa had a lower electron carrierconcentration of 1×10¹⁶/cm³.

In the obtained thin film, the electron mobility was higher than 1cm²/V·sec as shown in FIG. 2. However, the film deposited by the pulselaser vapor deposition at an oxygen partial pressure of higher than 6.5Pa as in this Example has a rough surface, being not suitable for achannel layer of the TFT.

Accordingly, a normally-off type transistor can be constructed by usinga transparent thin amorphous oxide represented by InGaO₃(ZnO)_(m) (m: anumber less than 6) in a crystal state formed at an oxygen partialpressure of higher than 4.5 Pa, preferably higher than 5 Pa, but lowerthan 6.5 Pa by a pulse laser vapor deposition method in the aboveExample.

The above obtained thin film exhibited an electron mobility higher than1 cm²/V, and the on-off ratio could be made higher than 1×10³.

As described above, in formation of an InGaZn oxide film by a PLD methodunder the conditions shown in this Example, the oxygen partial pressureis controlled in the range preferably from 4.5 Pa to 6.5 Pa.

For achieving the electron carrier concentration of 1×10¹⁸/cm³, theoxygen partial pressure conditions, the constitution of the filmformation apparatus, the kind and composition of the film-formingmaterial should be controlled.

Next, a top-gate type MISFET element as shown in FIG. 5 was produced byforming an amorphous oxide with the aforementioned apparatus at anoxygen partial pressure of 6.5 Pa. Specifically, on glass substrate 1, asemi-insulating amorphous InGaO₃(ZnO)₄ film of 120 nm thick was formedfor use for channel layer 2 by the above method of formation ofamorphous thin Ga—Ga—Zn—O film. Further thereon an InGaO₃(ZnO)₄ filmhaving a higher electro conductivity and a gold film were laminatedrespectively in a thickness of 30 nm by pulse laser deposition at anoxygen partial pressure of lower than 1 Pa in the chamber. Then drainterminal 5 and source terminal 6 were formed by photolithography and alift-off method. Finally, a Y₂O₃ film for gate insulator 3 was formed byan electron beam vapor deposition method (thickness: 90 nm, relativedielectric constant: about 15, leak current density: 1×10⁻³ A/cm³ atapplication of 0.5 MV/cm). Thereon, a gold film was formed, and gateterminal 4 was formed by photolithography and lifting-off.

Evaluation of Characteristics of MISFET Element

FIG. 6 shows current-voltage characteristics of the MISFET elementmeasured at room temperature. The channel is understood to be an n-typesemiconductor from the increase of the drain current I_(DS) with theincrease of the drain voltage V_(DS). This is consistent with the factthat an amorphous In—Ga—Zn—O type semiconductor is of an n-type. TheI_(DS) becomes saturated (pinched off) at V_(DS)=6V, which is typicalbehavior of a semiconductor transistor. From examination of the gaincharacteristics, the threshold value of the gate voltage V_(GS) underapplication of V_(DS)=4V was found to be about −0.5 V. A current flow ofI_(DS)=1.0×10⁻⁵ A was caused at V_(G)=10V. This corresponds to carrierinduction by gate bias in the In—Ga—Zn—O type amorphous semiconductorthin film.

The on-off ratio of the transistor was higher than 1×10³. From theoutput characteristics, the field effect mobility was calculated to beabout 7 cm²(Vs)⁻¹. Irradiation of visible light did not change thetransistor characteristics of the produced element according to the samemeasurement.

According to the present invention, a thin film transistor can beproduced which has a channel layer containing electron carriers at alower concentration to achieve higher electric resistance and exhibitinga higher electron mobility.

The above amorphous oxide has excellent characteristics that theelectron mobility increases with the increase of the electron carrierconcentration, and exhibits degenerate conduction. In this Example, thethin film was formed on a glass substrate. However, a plastic plate orfilm is useful as the substrate since the film formation can beconducted at room temperature. Further, the amorphous oxide obtained inthis Example, absorbs visible light only little to give transparentflexible TFT.

(Second Process for Film Formation: Sputtering Process (SP Process))

Film formation by a high-frequency SP process by use of an argon gas asthe atmosphere gas is explained below.

The SP process was conducted by use of the apparatus shown in FIG. 15.In FIG. 15, the numerals indicates the followings: 807, a substrate forfilm formation; 808, a target; 805, a substrate-holding means equippedwith a cooling mechanism; 814, a turbo molecular pump; 815, a rotarypump; 817, a shutter; 818, an ion manometer; 819, a Pirani gage; 821, agrowth chamber; and 830, a gate valve.

Substrate 807 for film formation was an SiO₂ glass substrate (CorningCo.: 1737) which had been washed ultrasonically for defatting withacetone, ethanol, and ultrapure water respectively for 5 minutes, anddried at 100° C. in the air.

The target was a polycrystalline sintered compact having a compositionof InGaO₃(ZnO)₄ (size: 20 nm diameter, 5 mm thick), which had beenprepared by wet-mixing In₂O₃, Ga₂O₃, and ZnO (each 4N reagent) as thesource material (solvent: ethanol), calcining the mixture (1000° C., 2hours), dry-crushing it, and sintering (1550° C., 2 hours). Target 808had an electro conductivity of 90 S/cm, being semi-insulating.

The final vacuum degree of growth chamber 821 was 1×10⁻⁴ Torr. Duringthe growth, the total pressure of the oxygen and argon gas was keptconstant within the range of 4 to 0.1×10⁻¹ Pa. The partial pressureratio of argon to oxygen was changed in the range of the oxygen partialpressure from 1×10⁻³ to 2×10⁻¹ Pa.

The substrate temperature was room temperature. The distance betweentarget 808 and substrate 807 for film formation was 30 mm.

The inputted electric power was RF 180 W, and the film forming rate was10 nm/min.

The resulting thin film was examined by small angle X-ray scatteringmethod (SAXS) (thin film method, incidence angle: 0.5°): no cleardiffraction peak was observed. Thus the obtained In—Ga—Zn—O type thinfilm was judged to be amorphous. From X-ray reflectivity and its patternanalysis, the mean square roughness (Rrms) was found to be about 0.5 nm,and the film thickness to be about 120 nm. From fluorescence X-rayspectrometric analysis (XRF), the metal composition of the film wasfound to be In:Ga:Zn=0.98:1.02:4.

The films were formed at various oxygen partial pressure of theatmosphere, and the resulting amorphous oxide films were measured forelectric conductivity. FIG. 3 shows the result.

As shown in FIG. 3, the electric conductivity can be lowered to lessthan 10 S/cm by conducting the film formation in an atmosphere having anoxygen partial pressure higher then 3×10⁻² Pa. The electron carriernumber could be decreased by increase of the oxygen partial pressure.

As shown in FIG. 3, for instance, the thin InGaO₃(ZnO)₄ film formed atthe substrate temperature of 25° C. and the oxygen partial pressure or1×10⁻¹ Pa had a lower electric conductivity of about 1×10⁻¹⁰ S/cm.Further, the thin InGaO₃(ZnO)₄ film formed at the oxygen partialpressure or 1×10⁻¹ Pa had an excessively high electric resistance,having the electric conductivity not measurable. With this film,although the electron mobility was not measurable, the electron mobilitywas estimated to be about 1 cm²/V·sec by extrapolation from the valuesof the films of high electron carrier concentration.

Thus, a normally-off transistor having the on-off ratio of higher than1×10³ could be obtained by use of a transparent thin amorphous oxidefilm constituted of In—Ga—Zn—O represented in a crystal state byInGaO₃(ZnO)_(m) (m: a natural number of less than 6) produced bysputtering vapor deposition in an argon atmosphere containing oxygen ata partial pressure of higher than 3×10⁻² Pa, preferably higher than5×10⁻¹ Pa.

In use of the apparatus and the material employed in this Example, thefilm formation by sputtering is conducted in the oxygen partial pressureranging from 3×10⁻² Pa to 5×10⁻¹ Pa. Incidentally, in the thin filmproduced by pulse laser vapor deposition or sputtering, the electronmobility increases with increase in number of the conductive electrons,as shown in FIG. 2.

As described above, by controlling the oxygen partial pressure, theoxygen defect can be decreased, and thereby the electron carrierconcentration can be decreased. In the amorphous thin film, the electronmobility can be high, since no grain interface exists essentially in theamorphous state differently from polycrystalline state.

Incidentally, the substitution of the glass substrate by a 200 μm-thickpolyethylene terephthalate (PET) film did not change the properties ofthe amorphous oxide film of InGaO₃(ZnO)₄ formed thereon.

A high-resistance amorphous film InGaO₃(Zn_(1-x)Mg_(x)O)_(m) (m: annatural number less than 6; 0<x≦1) can be obtained by using, as thetarget, polycrystalline InGaO₃(Zn_(1-x)Mg_(x)O)_(m) even at an oxygenpartial pressure less than 1 Pa. For instance, with a target in which 80atom % of Zn is replaced by Mg, the electron carrier concentration lowerthan 1×10¹⁶/cm (resistance: about 1×10⁻² S/cm) can be achieved by pulselaser deposition in an atmosphere containing oxygen at a partialpressure of 0.8 Pa. In such a film, the electron mobility is lower thanthat of the Mg-free film, but the decrease is slight: the electronmobility is about 5 cm²/V·sec at room temperature, being higher by aboutone digit than that of amorphous silicon. When the films are formedunder the same conditions, increase of the Mg content decreases both theelectric conductivity and the electron mobility. Therefore, the contentof the Mg ranges preferably from 20% to 85% (0.2<x<0.85).

In the thin film transistor employing the above amorphous oxide film,the gate insulator contains preferably a mixed crystal compoundcontaining two or more of Al₂O₃, Y₂O₃, HfO₂, and compounds thereof.

The presence of a defect at the interface between the gate-insulatingthin film and the channel layer thin film lowers the electron mobilityand causes hysteresis of the transistor characteristics. Moreover, thecurrent leakage depends greatly on the kind of the gate insulator.Therefore the gate insulator should be selected to be suitable for thechannel layer. The current leakage can be decreased by use of an Al₂O₃film, the hysteresis can be made smaller by use of a Y₂O₃ film, and theelectron mobility can be increased by use of an HfO₂ film having a highdielectric constant. By use of the mixed crystal of the above compounds,TFT can be formed which causes smaller current leakage, less hysteresis,and exhibiting a higher electron mobility. Since the gate insulatorforming process and the channel layer forming process can be conductedat room temperature, the TFT can be formed in a stagger constitution orin a reversed stagger constitution.

The TFT thus formed is a three-terminal element having a gate terminal,a source terminal, and a drain terminal. This TFT is formed by forming asemiconductor thin film on a insulating substrate of a ceramics, glass,or plastics as a channel layer for transport of electrons or holes, andserves as an active element having a function of controlling the currentflowing through the channel layer by application of a voltage to thegate terminal, and switching the current between the source terminal andthe drain terminal.

In the present invention, it is important that an intended electroncarrier concentration is achieved by controlling the amount of theoxygen defect.

In the above description, the amount of the oxygen in the amorphousoxide film is controlled by controlling the oxygen concentration in thefilm-forming atmosphere. Otherwise the oxygen defect quantity can becontrolled (decreased or increase) by post-treatment of the oxide filmin an oxygen-containing atmosphere as a preferred embodiment.

For effective control of the oxygen defect quantity, the temperature ofthe oxygen-containing atmosphere is controlled in the range from 0° C.to 300° C., preferably from 25° C. to 250° C., more preferably from 100°C. to 200° C.

Naturally, a film may be formed in an oxygen-containing atmosphere andfurther post-treated in an oxygen-containing atmosphere. Otherwise thefilm is formed without control of the oxygen partial pressure andpost-treatment is conducted in an oxygen-containing atmosphere, insofaras the intended electron carrier concentration (less than 1×10¹⁸/cm³)can be achieved.

The lower limit of the electron carrier concentration in the presentinvention is, for example, 1×10¹⁴/cm³, depending on the kind of theelement, circuit, or device employing the produced oxide film.

(Broader Range of Materials)

After investigation on other materials for the system, it was found thatan amorphous oxide composed of at least one oxide of the elements of Zn,In, and Sn is useful for an amorphous oxide film of a low carrierconcentration and high electron mobility. This amorphous oxide film wasfound to have a specific property that increase in number of conductiveelectrons therein increases the electron mobility. Using this film, anormally-off type TFT can be produced which is excellent in transistorproperties such as the on-off ratio, the saturation current in thepinch-off state, and the switching rate.

In the present invention, an oxide having any one of the characteristicsof (a) to (h) below are useful:

(a) An amorphous oxide which has an electron carrier concentration lessthan 1×10¹⁸/cm³;(b) An amorphous oxide in which the electron mobility becomes increasedwith increase of the electron carrier concentration;(The room temperature signifies a temperature in the range from about 0°C. to about 40° C. The term “amorphous compound” signifies a compoundwhich shows a halo pattern only without showing a characteristicdiffraction pattern in X-ray diffraction spectrum. The electron mobilitysignifies the one measured by the Hall effect.)(c) An amorphous oxide mentioned in the above items (a) or (b), in whichthe electron mobility at room temperature is higher than 0.1 cm²/V·sec;(d) An amorphous oxide mentioned, in any of the items (b) to (c), whichshows degenerate conduction;(The term “degenerate conduction” signifies the state in which thethermal activation energy in temperature dependency of the electricresistance is not higher than 30 meV.)(e) An amorphous oxide, mentioned in any of the above item (a) to (d),which contains at least one of the elements of Zn, In, and Sn as theconstituting element;(f) An amorphous oxide film composed of the amorphous oxide mentionedthe above item (e), and additionally at least one of the elements ofGroup-2 elements M2 having an atomic number lower than Zn (Mg, and Ca),Group-3 elements M3 having an atomic number lower than In (B, Al, Ga,and Y),Group-4 elements M4 having an atomic number lower than Sn (Si, Ge, andZr),Group-5 elements M5 (V, Nb, and Ta), andLu, and W to lower the electron carrier concentration;(g) An amorphous oxide film, mentioned in any of the above items (a) to(f), constituted of a single compound having a composition ofIn_(2-x)M3_(x)O₃(Zn_(1-y)M2_(y)O)_(m) (0≦x≦1; 0≦y≦1; m: 0 or a naturalnumber of less than 6) in a crystal state, or a mixture of the compoundsdifferent in number m, an example of M3 being Ga, and an example of M2being Mg; and(h) An amorphous oxide film, mentioned in any of the above items (a) to(g) formed on a plastic substrate or an plastic film.

The present invention also provides a field-effect transistor employingthe above mentioned amorphous oxide or amorphous oxide film as thechannel layer.

A field-effect transistor is prepared which is employs an amorphousoxide film having an electron carrier concentration of less than1×10¹⁸/cm³ but more than 1×10¹⁵/cm³ as the channel layer, and having asource terminal and a drain terminal, and a gate terminal withinterposition of a gate insulator. When a voltage of about 5 V isapplied between the source and drain terminals without application ofgate voltage, the electric current between the source and drainterminals is about 1×10⁻⁷ amperes.

The electron mobility in the oxide crystal becomes larger with increaseof the overlap of the s-orbitals of the metal ions. In an oxide crystalof Zn, In, or Sn having a higher atomic number, the electron mobility isin the range from 0.1 to 200 cm²/V·sec.

In an oxide, oxygen and metal ions are bonded by ionic bonds withoutorientation of the chemical bonds, having a random structure. Thereforein the oxide in an amorphous state, the electron mobility can becomparable to that in a crystal state.

On the other hand, substitution of the Zn, In, or Sn with an element ofa lower atomic number decreases the electron mobility. Thereby theelectron mobility in the amorphous oxide of the present invention rangesfrom about 0.01 to 20 cm²/V·sec.

In the transistor having a channel layer constituted of the above oxide,the gate insulator is preferably formed from Al₂O₃, Y₂O₃, HfO₂, or amixed crystal compound containing two or more thereof.

The presence of a defect at the interface between the gate-insulatingthin film and the thin channel layer film lowers the electron mobilityand causes hysteresis of the transistor characteristics. Moreover, thecurrent leakage depends greatly on the kind of the gate insulator.Therefore the gate insulator should be selected to be suitable for thechannel layer. The current leakage can be decreased by use of an Al₂O₃film, the hysteresis can be made smaller by use of a Y₂O₃ film, and theelectron mobility can be increased by use of an HfO₂ film having a highdielectric constant. By use of the mixed crystal of the above compounds,TFT can be formed which causes smaller current leakage, less hysteresis,and exhibiting a higher electron mobility. Since the gateinsulator-forming process and the channel layer-forming process can beconducted at room temperature, the TFT can be formed in a staggerconstitution or in a reversed stagger constitution.

The In₂O₃ oxide film can be formed through a gas-phase process, andaddition of moisture in a partial pressure of about 0.1 Pa to thefilm-forming atmosphere makes the formed film amorphous.

ZnO and SnO₂ respectively cannot readily be formed in an amorphous filmstate. For formation of the ZnO film in an amorphous state, In₂O₃ isadded in an amount of 20 atom %. For formation of the SnO₂ film in anamorphous state, In₂O₃ is added in an amount of 90 atom %. In formationof Sn—In—O type amorphous film, gaseous nitrogen is introduced in apartial pressure of about 0.1 Pa in the film formation atmosphere.

To the above amorphous film, may be added an element capable of forminga complex oxide, selected from Group-2 elements M2 having an atomicnumber lower than Zn (Mg, and Ca), Group-3 elements M3 having an atomicnumber lower than In (B, Al, Ga, and Y), Group-4 elements M4 having anatomic number lower than Sn (Si, Ge, and Zr), Group-5 elements M5 (V,Nb, and Ta), and Lu, and W. The addition of the above element stabilizesthe amorphous film at room temperature, and broadens the compositionrange for amorphous film formation.

In particular, addition of B, Si, or Ge tending to form a covalent bondis effective for amorphous phase stabilization. Addition of a complexoxide constituted of ions having largely different ion radiuses iseffective for amorphous phase stabilization. For instance, in an In—Zn—Osystem, for formation of a film stable at room temperature, In should becontained more than about 20 atom %. However, addition of Mg in anamount equivalent to In enables formation of stable amorphous film inthe composition range of In of not less than about 15 atom %.

In a gas-phase film formation, an amorphous oxide film of the electroncarrier concentration ranging from 1×10¹⁵/cm³ to 1×10¹⁸/cm³ can beobtained by controlling the film forming atmosphere.

An amorphous oxide film can be suitably formed by a vapor phase processsuch as a pulse laser vapor deposition process (PLD process), asputtering process (SP process), and an electron-beam vapor deposition.Of the vapor phase processes, the PLD process is suitable in view ofease of material composition control, whereas the SP process is suitablein view of the mass production. However, the film-forming process is notlimited thereto.

(Formation of In—Zn—Ga—O Type Amorphous Oxide Film by PLD Process)

An In—Zn—Ga—O type amorphous oxide was deposited on a glass substrate(Corning Co.: 1737) by a PLD process employing a KrF excimer laser witha polycrystal sintered compact as the target having a composition ofInGaO₃(ZnO) or InGaO₃(ZnO)₄.

The apparatus shown in FIG. 14 was employed which is mentioned before,and the film formation conditions were the same as mentioned before forthe apparatus.

The substrate temperature was 25° C.

The resulting two thin films were examined by small angle X-rayscattering method (SAXS) (thin film method, incidence angle: 0.5°): noclear diffraction peak was observed, which shows that the obtainedIn—Ga—Zn—O type thin films produced with two different targets were bothamorphous.

From X-ray reflectivity of the In—Zn—Ga—O type amorphous oxide film ofthe glass substrate and its pattern analysis, the mean squareroughnesses (Rrms) of the thin films were found to be about 0.5 nm, andthe film thicknesses to be about 120 nm. From fluorescence X-rayspectrometric analysis (XRF), the film obtained with the target of thepolycrystalline sintered compact of InGaO₃(ZnO) was found to contain themetals at a composition ratio In:Ga:Zn=1.1:1.1:0.9, whereas the filmobtained with the target of the polycrystalline sintered compact ofInGaO₃(ZnO)₄ was found to contain the metals at a composition ratioIn:Ga:Zn=0.98:1.02:4.

Amorphous oxide films were formed at various oxygen partial pressure ofthe film-forming atmosphere with the target having the composition ofInGaO₃(ZnO)₄. The formed amorphous oxide films were measured for theelectron carrier concentration. FIG. 1 shows the results. By formationof the film in an atmosphere having an oxygen partial pressure of higherthan 4.2 Pa, the electron carrier concentration could be lowered to lessthan 1×10¹⁸/cm³ as shown in FIG. 1. In this film formation, thesubstrate was kept nearly at room temperature without intentionalheating. At the oxygen partial pressure of lower than 6.5 Pa, thesurfaces of the obtained amorphous oxide films were flat.

At the oxygen partial pressure of 5 Pa, in the amorphous film formedwith the InGaO₃(ZnO)₄ target, the electron carrier concentration was1×10¹⁶/cm³, the electro conductivity was 1×10⁻² S/cm, and the electronmobility therein was estimated to be about 5 cm²/V·sec. From theanalysis of the light absorption spectrum, the optical band gap energybreadth of the formed amorphous oxide film was estimated to be about 3eV.

The higher oxygen partial pressure further lowered the electron carrierconcentration. As shown in FIG. 1, in the In—Zn—Ga—O type amorphousoxide film formed at a substrate temperature of 25□ at an oxygen partialpressure of 6 Pa, the electron carrier concentration was lowered to8×10¹⁵/cm³ (electroconductivity: about 8×10⁻³ S/cm). The electronmobility in the film was estimated to be 1 cm²/V·sec or more. However,by the PLD process, at the oxygen partial pressure of 6.5 Pa or higher,the deposited film has a rough surface, being not suitable for use asthe channel layer of the TFT.

The In—Zn—Ga—O type amorphous oxide films were formed at various oxygenpartial pressures in the film-forming atmosphere with the targetconstituted of a polycrystalline sintered compact having the compositionof InGaO₃(ZnO)₄. The resulting films were examined for the relationbetween the electron carrier concentration and the electron mobility.FIG. 2 shows the results. Corresponding to the increase of the electroncarrier concentration from 1×10¹⁶/cm³ to 1×10²⁰/cm³, the electronmobility increased from about 3 cm²/V·sec to about 11 cm²/V·sec. Thesame tendency was observed with the amorphous oxide films obtained withthe polycrystalline sintered InGaO₃(ZnO) target.

The In—Zn—Ga—O type amorphous oxide film which was formed on a 200μm-thick polyethylene terephthalate (PET) film in place of the glasssubstrate had similar characteristics.

(Formation of In—Zn—Ga—Mg—O Type Amorphous Oxide Film by PLD Process)

A film of InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x≦1) was formed on a glasssubstrate by a PLD process with an InGaO₃(Zn_(1-x)Mg_(x)O)₄ target(0<x≦1). The apparatus employed was the one shown in FIG. 14.

An SiO₂ glass substrate (Corning Co.: 1737) was used as the substrate.As the pretreatment, the substrate was washed ultrasonically fordefatting with acetone, ethanol, and ultrapure water each for fiveminutes, and dried in the air at 100° C. The target was a sinteredcompact of InGaO₃(Zn_(1-x)Mg_(x)O)₄ (x=1-0) (size: 20 mm diameter, 5 mmthick).

The target was prepared by wet-mixing source materials In₂O₃, Ga₂O₃, andZnO (each 4N reagent) (solvent: ethanol), calcining the mixture (1000°C., 2 hours), dry-crushing it, and sintering it (1550° C., 2 hours). Thefinal pressure in the growth chamber was 2×10⁻⁶ Pa. The oxygen partialpressure during the growth was controlled at 0.8 Pa. The substratetemperature was room temperature (25° C.). The distance between thetarget and the substrate for film formation was 30 mm. The KrF excimerlaser was irradiated at a power of 1.5 mJ/cm²/pulse with the pulse widthof 20 nsec, the repeating frequency of 10 Hz, and the irradiation spotsize of 1×1 mm square. The film-forming rate was 7 nm/min. The oxygenpartial pressure in the film-forming atmosphere was 0.8 Pa. Thesubstrate temperature was 25° C.

The resulting thin film was examined by small angle X-ray scatteringmethod (SAXS) (thin film method, incidence angle: 0.5°): no cleardiffraction peak was observed. Thus the obtained In—Ga—Zn—Mg—O type thinfilm was amorphous. The resulting film had a flat surface.

By using targets of different x-values (different Mg content),In—Zn—Ga—Mg—O type amorphous oxide films were formed at the oxygenpartial pressure of 0.8 Pa in a film-forming atmosphere to investigatethe dependency of the conductivity, the electron carrier concentration,and the electron mobility on the x-value.

FIGS. 4A, 4B, and 4C show the results. At the x values higher than 0.4,in the amorphous oxide films formed by the PLD process at the oxygenpartial pressure of 0.8 Pa in the atmosphere, the electron carrierconcentration was decreased to be less than 1×10¹⁸/cm³. In the amorphousfilm of x value higher than 0.4, the electron mobility was higher than 1cm²/V.

As shown in FIGS. 4A, 4B, and 4C, the electron carrier concentrationless than 1×10¹⁶/cm³ could be achieved in the film prepared by a pulselaser deposition process with the target in which 80 atom % of Zn isreplaced by Mg and at the oxygen partial pressure of 0.8 Pa (electricresistance: about 1×10⁻² S·cm). In such a film, the electron mobility isdecreased in comparison with the Mg-free film, but the decrease isslight. The electron mobility in the films is about 5 cm²/V·sec, whichis higher by about one digit than that of amorphous silicon. Under thesame film formation conditions, both the electric conductivity and theelectron mobility in the film decrease with increase of the Mg content.Therefore, the Mg content in the film is preferably more than 20 atom %and less than 85 atom % (0.2<x<0.85), more preferably 0.5<x<0.85.

The amorphous film of InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x≦1) formed on a 200μm-thick polyethylene terephthalate (PET) film in place of the glasssubstrate had similar characteristics.

(Formation of In₂O₃ Amorphous Oxide Film by PLD Process)

An In₂O₃ film was formed on a 200 μm-thick PET film by use of a targetconstituted of In₂O₃ polycrystalline sintered compact by a PLD processemploying a KrF excimer laser.

The apparatus used is shown in FIG. 14. The substrate for the filmformation was an SiO₂ glass substrate (Corning Co.: 1737).

As the pretreatment before the deposition, the substrate was washedultrasonically for defatting with acetone, ethanol, and ultrapure watereach for five minutes, and dried in the air at 100° C.

The target was an In₂O₃ sintered compact (size: 20 mm diameter, 5 mmthick), which had been prepared by calcining the source material In₂O₃(4N reagent) (1000° C., 2 hours), dry-crushing it, and sintering it(1550° C., 2 hours).

The final vacuum of the growth chamber was 2×10⁻⁶ Pa, the oxygen partialpressure during the growth was 5 Pa, and the substrate temperature was25° C.

The water vapor partial pressure was 0.1 Pa, and oxygen radicals weregenerated by the oxygen radical-generating assembly by application of200 W.

The distance between the target and the film-holding substrate was 40mm, the power of the Krf excimer laser was 0.5 mJ/cm²/pulse, the pulsewidth was 20 nsec, the repeating frequency was 10 Hz, and theirradiation spot size was 1×1 mm square.

The film-forming rate was of 3 nm/min.

The resulting thin film was examined by small angle X-ray scatteringmethod (SAXS) (thin film method, incidence angle: 0.5°): no cleardiffraction peak was observed, which shows that the obtained In—O typeoxide film was amorphous. The film thickness was 80 nm.

In the obtained In—O type amorphous oxide film, the electron carrierconcentration was 5×10¹⁷/cm³, and the electron mobility was about 7cm²/V·sec.

(Formation of In—Sn—O Type Amorphous Oxide Film by PLD Process)

An In—Sn—O type oxide film was formed on a 200 μm-thick PET film by useof a target constituted of polycrystalline sintered compact of(In_(0.9)Sn_(0.1)) O_(3.1) by a PLD process employing a KrF excimerlaser. The apparatus used is shown in FIG. 14.

The substrate for the film formation was an SiO₂ glass substrate(Corning Co.: 1737).

As the pretreatment before the deposition, the substrate was washedultrasonically for defatting with acetone, ethanol, and ultrapure watereach for five minutes, and dried in the air at 100° C.

The target was an In₂O₃—SnO₂ sintered compact (size: 20 mm diameter, 5mm thick), which had been prepared by wet-mixing the source materialsIn₂O₃—SnO₂ (4N reagents) (solvent: ethanol), calcining the mixture(1000° C., 2 hours), dry-crushing it, and sintering it (1550° C., 2hours).

The substrate was kept at room temperature. The oxygen partial pressurewas 5 Pa. The nitrogen partial pressure was 0.1 Pa. Oxygen radicals weregenerated by the oxygen radical-generating assembly by application of200 W.

The distance between the target and the film-holding substrate was 30mm, the power of the Krf excimer laser was 1.5 mJ/cm²/pulse, the pulsewidth was 20 nsec, the repeating frequency was 10 Hz, and theirradiation spot size was 1×1 mm square.

The film-forming rate was of 6 nm/min.

The resulting thin film was examined by small angle X-ray scatteringmethod (SAXS) (thin film method, incidence angle: 0.5°): no cleardiffraction peak was detected, which shows that the obtained In—Sn—Otype oxide film is amorphous.

In the obtained In—Sn—O type amorphous oxide film, the electron carrierconcentration was 8×10¹⁷/cm³, and the electron mobility was about 5cm²/V·sec. The film thickness was 100 nm.

(Formation of In—Ga—O Type Amorphous Oxide Film by PLD Process)

The substrate for the film was an SiO₂ glass substrate (Corning Co.:1737).

As the pretreatment before the deposition, the substrate was washedultrasonically for defatting with acetone, ethanol, and ultrapure watereach for five minutes, and dried in the air at 100° C.

The target was a sintered compact of (In₂O₃)_(1-x)—(Ga₂O₃)_(x) (x=0-1)(size: 20 mm diameter, 5 mm thick). For instance, at x=0.1, the targetis a polycrystalline sintered compact of (In_(0.9)Ga_(0.1))₂O₃.

This target was prepared by wet-mixing the source materials In₂O₃—Ga₂O₂(4N reagents) (solvent: ethanol), calcining the mixture (1000° C., 2hours), dry-crushing it, and sintering it (1550° C., 2 hours).

The final pressure of the growth chamber was 2×10⁻⁶ Pa. The oxygenpartial pressure during the growth was 1 Pa.

The substrate was at room temperature. The distance between the targetand the film-holding substrate was 30 mm. The power of the Krf excimerlaser was 1.5 mJ/cm²/pulse. The pulse width was 20 nsec. The repeatingfrequency was 10 Hz. The irradiation spot size was 1×1 mm square. Thefilm-forming rate was of 6 nm/min.

The substrate temperature was 25° C. The oxygen pressure was 1 Pa. Theresulting film was examined by small angle X-ray scattering method(SAXS) (thin film method, incidence angle: 0.5°): no clear diffractionpeak was detected, which shows that the obtained In—Ga—O type oxide filmis amorphous. The film thickness was 120 nm.

In the obtained In—Ga—O type amorphous oxide film, the electron carrierconcentration was 8×10¹⁶/cm³, and the electron mobility was about 1cm²/V·sec.

(Preparation of TFT Element Having In—Zn—Ga—O type Amorphous Oxide Film(Glass Substrate))

A top gate type TFT element shown in FIG. 5 was prepared.

Firstly, an In—Ga—Zn—O type amorphous oxide film was prepared on glasssubstrate 1 by the aforementioned PLS apparatus with a targetconstituted of a polycrystalline sintered compact having a compositionof InGaO₃(ZnO)₄ at an oxygen partial pressure of 5 Pa. The formedIn—Ga—Zn—O film had a thickness of 120 nm, and was used as channel layer2.

Further thereon, another In—Ga—Zn—O type amorphous film having a higherelectro conductivity and a gold layer were laminated, each in 30 nmthick, by the PLD method at the oxygen partial pressure of lower than 1Pa in the chamber. Therefrom drain terminal 5 and source terminal 6 wereformed by photolithography and a lift-off method.

Finally, a Y₂O₃ film as gate insulator 3 was formed by electron beamvapor deposition (thickness: 90 nm, relative dielectric constant: about15, leakage current density: 1×10⁻³ A/cm² under application of 0.5MV/cm). Further thereon, a gold film was formed and therefrom gateterminal 4 was formed by photolithography and a lift-off method. Thechannel length was 50 μm, and the channel width was 200 μm.

Evaluation of Characteristics of TFT Element

FIG. 6 shows current-voltage characteristics of the TFT element at roomtemperature. Drain current I_(DS) increased with increase of drainvoltage V_(DS), which shows that the channel is of an n-type conduction.

This is consistent with the fact that an amorphous In—Ga—Zn—O typesemiconductor is of an n-type. The I_(DS) become saturated (pinched off)at V_(DS)=6V, which is typical behavior of a semiconductor transistor.From examination of the gain characteristics, the threshold value of thegate voltage V_(GS) under application of V_(DS)=4V was found to be about−0.5 V. A current flow of I_(DS)=1.0×10⁻⁵ A was cased at V_(G)=10V. Thiscorresponds to carrier induction by a gate bias in the In—Ga—Zn—O typeamorphous semiconductor thin film as the insulator.

The on-off ratio of the transistor was higher than 1×10³. From theoutput characteristics, the field effect mobility was calculated to beabout 7 cm²(Vs)⁻¹ in the saturation region. Irradiation of visible lightdid not change the transistor characteristics of the produced elementaccording to the same measurement.

The amorphous oxide of the electron carrier concentration lower than1×10¹⁸/cm³ is useful as a channel layer of a TFT. The electron carrierconcentration is more preferably less than 1×10¹⁷/cm³, still morepreferably less than 1×10¹⁶/cm³.

(Preparation of TFT Element Having In—Zn—Ga—O type Amorphous Oxide Film(Amorphous Substrate))

A top gate type TFT element shown in FIG. 5 was prepared.

Firstly, an In—Ga—Zn—O type amorphous oxide film was prepared onpolyethylene terephthalate (PET) film 1 by the aforementioned PLSapparatus with a target constituted of a polycrystalline sinteredcompact having a composition of InGaO₃(ZnO) at an oxygen partialpressure of 5 Pa in the atmosphere. The formed film had a thickness of120 nm, and was used as channel layer 2.

Further thereon, another In—Ga—Zn—O type amorphous film having a higherelectro conductivity and a gold layer were laminated, each in 30 nmthick, by the PLD method at the oxygen partial pressure of lower than 1Pa in the chamber. Therefrom drain terminal 5 and source terminal 6 wereformed by photolithography and a lift-off method.

Finally, gate insulator 3 was formed by an electron beam vapordeposition method. Further thereon, a gold film was formed and therefromgate terminal 4 was formed by photolithography and a lift-off method.The channel length was 50 μm, and the channel width was 200 μm. ThreeTFTs of the above structure were prepared by using respectively one ofthe three kinds of gate insulators: Y₂O₃ (140 nm thick), Al₂O₃ (130 μmthick), and HfO₂ (140 μm thick).

Evaluation of Characteristics of TFT Element

The TFT elements formed on a PET film had current-voltagecharacteristics similar to that shown in FIG. 6 at room temperature.Drain current I_(DS) increased with increase of drain voltage V_(DS),which shows that the channel is of an n-type conduction. This isconsistent with the fact that an amorphous In—Ga—Zn—O type semiconductoris of an n type. The I_(DS) become saturated (pinched off) at V_(DS)=6V,which is typical behavior of a semiconductor transistor. A current flowof I_(DS)=1.0×10⁻⁸ A was caused at V_(G)=0 V, and a current flow ofI_(DS)=2.0×10⁻⁵ A was caused at V_(G)=10 V. This corresponds to carrierinduction by gate bias in the insulator, the In—Ga—Zn—O type amorphoussemiconductor oxide film.

The on-off ratio of the transistor was higher than 1×10³. From theoutput characteristics, the field effect mobility was calculated to beabout 7 cm²(Vs)⁻¹ in the saturation region.

The elements formed on the PET film were curved at a curvature radius of30 mm, and in this state, transistor characteristics were measured.However the no change was observed in the transistor characteristics.Irradiation of visible light did not change the transistorcharacteristics.

The TFT employing the Al₂O₃ film as the gate insulator has alsotransistor characteristics similar to that shown in FIG. 6. A currentflow of I_(DS)=1.0×10⁻⁸ A was caused at V_(G)=0 V, and a current flow ofI_(DS)=5.0×10⁻⁶ A was caused at V_(G)=10 V. The on-off ratio of thetransistor was higher than 1×10². From the output characteristics, thefield effect mobility was calculated to be about 2 cm²(Vs)⁻¹ in thesaturation region.

The TFT employing the HfO₂ film as the gate insulator has alsotransistor characteristics similar to that shown in FIG. 6. A currentflow of I_(DS)=1×10⁻⁸ A was caused at V_(G)=0 V, and a current flow ofI_(DS)=1.0×10⁻⁶ A was caused at V_(G)=10 V. The on-off ratio of thetransistor was higher than 1×10². From the output characteristics, thefield effect mobility was calculated to be about 10 cm²(Vs)⁻¹ in thesaturation region.

(Preparation of TFT Element Employing In₂O₃ Amorphous Oxide Film by PLDProcess)

A top gate type TFT element shown in FIG. 5 was prepared.

Firstly, an In₂O₃ type amorphous oxide film was formed on polyethyleneterephthalate (PET) film 1 by the PLD method as channel layer 2 in athickness of 80 nm.

Further thereon, another In₂O₃ amorphous film having a higher electroconductivity and a gold layer were laminated, each in 30 nm thick, bythe PLD method at the oxygen partial pressure of lower than 1 Pa in thechamber, and at the voltage application of zero volt to the oxygenradical generating assembly. Therefrom drain terminal 5 and sourceterminal 6 were formed by photolithography and a lift-off method.

Finally, a Y₂O₃ film as gate insulator 3 was formed by an electron beamvapor deposition method. Further thereon, a gold film was formed andtherefrom gate terminal 4 was formed by photolithography and a lift-offmethod.

Evaluation of Characteristics of TFT Element

The TFT elements formed on a PET film was examined for current-voltagecharacteristics at room temperature. Drain current I_(DS) increased withincrease of drain voltage V_(DS), which shows that the channel is ann-type conductor. This is consistent with the fact that an amorphousIn—O type amorphous oxide film is an n type conductor. The I_(DS) becomesaturated (pinched off) at about V_(DS)=6V, which is typical behavior ofa transistor. A current flow of I_(DS)=2×10⁻⁸ A was caused at V_(G)=0 V,and a current flow of I_(DS)=2.0×10⁻⁶ A was caused at V_(G)=10 V. Thiscorresponds to electron carrier induction by gate bias in the insulator,the In—O type amorphous oxide film.

The on-off ratio of the transistor was about 1×10². From the outputcharacteristics, the field effect mobility was calculated to be about1×10 cm² (Vs)⁻¹ in the saturation region. The TFT element formed on aglass substrate had similar characteristics.

The elements formed on the PET film were curved in a curvature radius of30 mm, and in this state, transistor characteristics were measured. Nochange was observed in the transistor characteristics.

(Preparation of TFT Element Employing In—Sn—O type Amorphous Oxide Filmby PLD Process)

A top gate type TFT element shown in FIG. 5 was prepared.

Firstly, an In—Sn—O type amorphous oxide film was formed in a thicknessof 100 nm as channel layer 2 on polyethylene terephthalate (PET) film 1by the PLD method.

Further thereon, another In—Sn—O amorphous film having a higher electroconductivity and a gold layer were laminated, each in 30 nm thick, bythe PLD method at the oxygen partial pressure lower than 1 Pa in thechamber, and at voltage application of zero volt to the oxygen radicalgenerating assembly. Therefrom drain terminal 5 and source terminal 6were formed by photolithography and a lift-off method.

Finally, a Y₂O₃ film as gate insulator 3 was formed by an electron beamvapor deposition method. Further thereon, a gold film was formed andtherefrom gate terminal 4 was formed by photolithography and a lift-offmethod.

Evaluation of Characteristics of TFT Element

The TFT elements formed on a PET film was examined for current-voltagecharacteristics at room temperature. Drain current I_(DS) increased withincrease of drain voltage V_(DS), which shows that the channel is ann-type conductor. This is consistent with the fact that an amorphousIn—Sn—O type amorphous oxide film is an n type conductor. The I_(DS)became saturated (pinched off) at about V_(DS)=6V, which is typicalbehavior of a transistor. A current flow of I_(DS)=5×10⁻⁸ A was causedat V_(G)=0 V, and a current flow of I_(DS)=5.0×10⁻⁵ A was caused atV_(G)=10 V. This corresponds to electron carrier induction by the gatebias in the insulator, the In—Sn—O type amorphous oxide film.

The on-off ratio of the transistor was about 1×10³. From the outputcharacteristics, the field effect mobility was calculated to be about 5cm² (Vs)⁻¹ in the saturation range. The TFT element formed on a glasssubstrate had similar characteristics.

The elements formed on the PET film were curved at a curvature radius of30 mm, and in this state, transistor characteristics were measured. Nochange was caused thereby in the transistor characteristics.

(Preparation of TFT Element Employing In—Ga—O type Amorphous Oxide Filmby PLD Process)

A top gate type TFT element shown in FIG. 5 was prepared.

Firstly, an In—Ga—O type amorphous oxide film was formed in a thicknessof 120 nm as channel layer 2 on polyethylene terephthalate (PET) film 1by the PLD method shown in Example 6.

Further thereon, another In—Ga—O amorphous film having a higherconductivity and a gold layer were laminated, each in 30 nm thick, bythe PLD method at the oxygen partial pressure of lower than 1 Pa in thechamber, and at the voltage application of zero volt to the oxygenradical-generating assembly. Therefrom drain terminal 5 and sourceterminal 6 were formed by photolithography and a lift-off method.

Finally, a Y₂O₃ film as gate insulator 3 was formed by an electron beamvapor deposition method. Further thereon, a gold film was formed andtherefrom gate terminal 4 was formed by photolithography and a lift-offmethod.

Evaluation of Characteristics of TFT Element

The TFT elements formed on a PET film was examined for current-voltagecharacteristics at room temperature. Drain current I_(DS) increased withincrease of drain voltage V_(DS), which shows that the channel is ann-type conductor. This is consistent with the fact that an amorphousIn—Ga—O type amorphous oxide film is an n type conductor. The I_(DS)became saturated (pinched off) at about V_(DS)=6V, which is typicalbehavior of a transistor. A current flow of I_(DS)=1×10⁻⁸ A was causedat V_(G)=0 V, and a current flow of I_(DS)=1.0×10⁻⁶ A was caused atV_(G)=10 V. This corresponds to electron carrier induction by the gatebias in the insulator, the In—Ga—O type amorphous oxide film.

The on-off ratio of the transistor was about 1×10². From the outputcharacteristics, the field effect mobility was calculated to be about0.8 cm² (Vs)⁻¹ in the saturation range. The TFT element formed on aglass substrate had similar characteristics.

The elements formed on the PET film were curved at a curvature radius of30 mm, and in this state, transistor characteristics were measured. Nochange was caused thereby in the transistor characteristics.

The amorphous oxide of the electron carrier concentration of lower than1×10¹⁸/cm³ is useful as the channel layer of the TFT. The electroncarrier concentration is more preferably not higher than 1×10¹⁷/cm³,still more preferably not higher than 1×10¹⁶/cm³.

An embodiment on a light-emitting device according to the presentinvention will be now shown below.

Example 1

An example of a light-emitting device will be described in detail withreference to an embodiment.

An amorphous In—Ga—Zn—O thin film was formed on a substrate so as toacquire a composition of In:Ga:Zn=0.98:1.02:4, by using the alreadydescribed PLD method.

A top gate type MISFET element shown in FIG. 5 was prepared by the stepsof:

at first, forming a semi-insulating amorphous InGaO₃(ZnO)₄ film into athickness of 120 nm on the glass substrate (1), which is used as achannel layer (2), with the method of having prepared the abovedescribed amorphous In—Ga—Zn—O thin film; layering an InGaO₃(ZnO)₄ filmwith high electroconductivity and a gold film respectively into thethickness of 30 nm further thereon with a pulsed laser deposition methodin a chamber with an oxygen partial pressure of less than 1 Pa, andforming a drain terminal (5) and a source terminal (6) with aphotolithographic method and a lift-off technology; and finally, forminga Y₂O₃ film which is used as a gate insulation film (3) with an electronbeam vacuum deposition method (thickness: 90 nm, relative permittivity:about 15, and leak current density: 1×10⁻³ A/cm² when 0.5 MV/cm isapplied), forming a gold film thereon, and forming a gate terminal (4)with the photolithographic method and the lift-off technology.

(Evaluation for Characteristics of MISFET Element)

FIG. 6 shows current/voltage characteristics of an MISFET elementmeasured at room temperature. When drain voltage V_(DS) increases, adrain current I_(DS) also increases, which shows that a channel is ann-type semiconductor. The result is not contradictory to a fact that anamorphous In—Ga—Zn—O-based semiconductor is an n-type. The MISFETelement showed the typical behavior of a semiconductor transistor inwhich an I_(DS) saturates (pinches off) at a V_(DS) of about 6 V. As aresult of having examined a gain characteristic, the threshold value ofgate voltage V_(GS) showed about −0.5 V when a V_(DS) of 4 V wasapplied. In addition, a current I_(DS) showed 1.0×10⁻⁵ amperes when aV_(G) of 10 V was applied. The result means that an In—Ga—Zn—O baseamorphous semiconductor thin film of an insulation material induced acarrier therein by a gate bias.

An ON/OFF ratio of a transistor exceeded 1×10³. In addition, as a resultof calculating electron field-effect mobility from outputcharacteristics, the electron field-effect mobility was about 7 cm²(Vs)⁻¹ in a saturation range. Similar measurement was performed on theprepared element while irradiating it with visible light, the elementdid not show any change of transistor characteristics. Herefrom, it isunderstood that the prepared element can be used as an opening also in abottom emission type, without needing to interrupt a transistor regionfrom light.

An MISFET element was formed with the approximately same method as theabove described method. But after the MISFET element was prepared, aninsulation film was formed into the thickness of 300 nm thereon with apulsed laser deposition method. At the same time, a contact hole forconnecting a drain or source terminal with a bottom electrode wasformed.

Subsequently, the film of Al was formed into the thickness of 300 nmwith a resistance heating vacuum deposition method, and the film of analloy of Al and Ag was formed into the thickness of 50 nm thereon for abottom electrode. The bottom electrode was connected with a drain orsource electrode through a contact hole.

Subsequently, an organic EL light-emitting layer was prepared by formingthe following films with a resistance vapor deposition method: the filmof tris(8-quinolinol) aluminum with the thickness of 20 nm as anelectron transport layer; the film of 4,4′-bis(2,2-diphenyl)vinyl withthe thickness of 40 nm as a light-emitting layer thereon; the film of4,4′-bis[1-(naphthyl)-N-phenylamino]biphenyl with the thickness of 20 nmas a hole transport layer; and the film of4,4′-bis[N,N-diamino]-4″-phenyl-triphenyl amine] with the thickness of60 nm as a hole injection layer.

Finally, the film of ITO was formed into the thickness of 200 nm for acounter electrode with a counter target sputtering technique.

When the above described element was driven by contacting a probethereon, blue light was emitted from a top surface of a substrate, inother words, a top emission type of the element was obtained.

Example 2 Bottom Emission Type

Subsequently, an example of a preparation process for a light-emittingdevice of a bottom emission type, in which a drain electrode and abottom electrode are directly connected, will be described withreference to FIG. 9.

An MISFET element was formed with the approximately same method as inthe above described Embodiment 1. But a drain electrode was not formed,and an active layer 92 was left to have the same area as that of alight-emitting layer. Subsequently, the film of InGaO₃(ZnO)₄ with highelectroconductivity was formed into the thickness of 200 nm thereon, asan electrode 97 serving as the drain (or source) electrode and a bottomelectrode, with a pulsed laser deposition method in a chamber having anoxygen partial pressure of less than 1 Pa.

Then, an organic EL light-emitting layer was formed by forming anorganic layer in a retrograde order of Embodiment 1 with a resistanceevaporation method, and the whole integrated layers were named as theorganic EL light-emitting layer.

Finally, the film of Al doped with Li was formed into the thickness of50 nm, and the film of Al into the thickness of 200 nm as a counterelectrode 99, with a resistance heating method.

When the above described element is driven by contacting a probethereon, blue light was emitted from a back side of a substrate, inother words, a bottom emission type of the element is obtained.

Example 3 Linearly Arrayed Light Source

A linearly arrayed light source according to the present invention willbe described with reference to FIG. 10.

A plurality of light-emitting elements 101 and thin film transistors(TFT) 102 were arranged on a substrate into a linear form, and wereelectrically connected as shown in FIG. 10. A linearly arrayed lightsource was prepared by connecting a control circuit 103 for controllinglight-emitting with the light-emitting elements and the TFTs. A TFToutput (a source electrode or a drain electrode) was connected with apower source Vd through the light-emitting element, and the other wasconnected with a common electric potential COM. In addition, the controlcircuit was connected to the gate electrode of the TFT.

When a TFT was turned on by a signal output from a control circuit, alight-emitting element emitted light. Specifically, the linearly arrayedlight source could emit a desired linear light-emitting pattern when asignal from the control circuit was appropriately controlled.

Here, the light-emitting element can employ an arbitrary light-emittingelement such as an organic electroluminescent element (an organic EL),an inorganic electroluminescent element (an inorganic EL) and a LED. Inaddition, a circuit configuration is not limited thereto, but aplurality of TFTs may be arranged with respect to one light-emittingelement.

Thus, a configuration of driving a light-emitting element by anamorphous-oxide TFT can be extremely easily manufactured, andinexpensively provide a linearly arrayed light source.

FIG. 11 shows a sectional view of an example having a configuration of alinearly arrayed light source according to the present invention.

As shown in FIG. 11, a linearly arrayed light source is prepared byarranging a TFT part and a light-emitting element section on asubstrate, and each element on each portion. FIG. 11 shows a case ofusing an organic EL element as a light-emitting element, and the abovedescribed amorphous oxide TFT in a TFT portion.

In the figure, reference numeral 2100 denotes a substrate, referencenumeral 2101 a channel layer, reference numeral 2102 a source electrode,reference numeral 2103 a gate insulation film, reference numeral 2104 agate electrode, reference numeral 2105 a drain electrode, referencenumeral 2106 an upper electrode (first electrode), reference numeral2107 an electron transport layer, reference numeral 2108 alight-emitting layer, reference numeral 2109 a hole transport layer,reference numeral 2110 a transparent electrode layer, reference numeral2111 an insulation layer, reference numeral 2150 a TFT section andreference numeral 2160 a light-emitting element section.

FIG. 11 shows an example of connecting drain electrode 2105 totransparent electrode layer 2110 of the light-emitting element section.However, there is also a case of connecting source electrode 2102 totransparent electrode layer 2110, depending on a layer constitution ofthe light-emitting layer.

As for a preparation method, the linearly arrayed light source wasprepared by the steps of forming an amorphous oxide TFT on apredetermined position on a substrate with the above describedtechnique, then an organic EL element, and further wiring each on otherpositions.

At this time, when having formed a gate oxide film of an amorphous oxideTFT, the same insulation layer was formed at a portion in which anorganic EL element would be arranged. The organic EL element wasprepared by placing a mask on a substrate, and sequentially forming atransparent electrode layer, subsequently, a hole transport layer, alight-emitting layer, an electron transport layer and an upper electrodelayer. During the above steps, the transparent electrode layer wasconnected to a drain electrode layer. Finally, various wiring was formedby an aluminum film or the like.

Here, an arbitrary transparent electroconductive film such as In₂O₃:Sncan be used as a transparent electrode layer. A material which is usedin a general organic EL element can be applied to a hole transportlayer, a light-emitting layer, an electron transport layer and an upperelectrode layer. For instance, α-NPD can be used as the hole transportlayer, CBP doped with 6% Ir (ppy)₃ as the light-emitting layer, Alq3 asthe electron transport layer and AgMg as the upper electrode layer.

In the above description,

Alq3 means an aluminum-quinolinol complex;

α-NPD; and

Ir (ppy)₃ means an iridium-phenyl pyridine complex.

In the above configuration, light emitted from a light-emitting elementpassed through a transparent electrode and was emitted from a substrateside, as is shown by an arrow 2170 in the figure.

Thus obtained linearly arrayed light source was small, lightweight andinexpensive.

As for another configuration of a linearly arrayed light source, therecan be the configuration of arranging an organic EL element on an upperpart of a TFT placed on a substrate as shown in FIG. 12. The abovelinearly arrayed light source can make light emitted from alight-emitting element pass through a TFT section and emanate from asubstrate side, by making an amorphous oxide TFT a transparent devicethrough preparing an electrode with a transparent electrode such asIn₂O₃:Sn.

In FIG. 12, reference numeral 2200 denotes a substrate, referencenumeral 2201 a channel layer, reference numeral 2202 a source electrode,reference numeral 2203 a gate insulation film, reference numeral 2204 agate electrode, reference numeral 2205 a drain electrode, referencenumeral 2206 an upper electrode (a first electrode), reference numeral2207 an electron transport layer, reference numeral 2208 alight-emitting layer, reference numeral 2209 a hole transport layer,reference numeral 2210 a transparent electrode layer, reference numeral2211 an insulation layer, reference numeral 2250 a TFT section andreference numeral 2260 a light-emitting element section.

FIG. 12 shows an example of connecting drain electrode 2205 totransparent electrode layer 2210 of the light-emitting element section.However, there is also a case of connecting source electrode 2202 totransparent electrode layer 2210, depending on a layer constitution ofthe light-emitting layer.

Thus formed configuration makes light emanate in the way as indicated byan arrow 2270, enables a substrate surface to be effectively used, andcan make a linearly arrayed light source arrange organic EL elementsthereon at high density, and/or acquire a large light-emitting area (anopen area ratio). Thus obtained linearly arrayed light source is small,lightweight and inexpensive.

Example 4 Application to Copying Machine and Page Printer

In the next place, a linearly arrayed light source applied to a copyingmachine or a page printer will be described.

Most of general copying machines or page printers have a system forprinting data on a draughting medium, of scanning a laser beam over aphotoconductor drum by using a lens and a polygon mirror to print thedata thereon, and recording the data on the photoconductor drum.

On the other hand, the above devices can be small and inexpensive ifemploying a linearly arrayed light source according to the presentinvention, because of directly irradiating a photoconductor drum withlight emitted from a light-emitting element without passing the lightthrough a lens system, as shown in FIG. 13. As a result, the abovedevices need no large-scaled optical system, and accordingly can beminiaturized and reduce the manufacturing cost. If necessary, the abovedevices may arrange a simple optical system such as SELFOC lens betweenthe light-emitting element and the drum.

In FIG. 13, reference numeral 2300 denotes a linearly arrayed lightsource, and reference numeral 2350 a photoconductor drum.

When preparing the above copying machine or a page printer, it ispreferable to unify the above linearly arrayed light source with aphotoconductor drum cartridge.

The linearly arrayed light source is inexpensive and accordinglydisposable, and as a result, can make the copying machine or the pageprinter maintenance-free through preparing a single-piece structurecombined with the photoconductor drum, while making use of the abovecharacteristics. The above described light source may be arrangedoutside the photoconductor (a photoconductor drum) as shown in thepresent embodiment or may also be arranged in the inside.

When an electronograph employs the linearly arrayed light sourceaccording to the present embodiment, the electronograph forms an imageby using an electrifier for electrifying the photoreceptor (aphotoconductor drum), an exposing light source for illuminating thephotoreceptor in order to form a latent image on the photoreceptor, anda developing device for developing the above described latent image; andemploys the linearly arrayed light source according to the presentinvention for the exposing light source.

INDUSTRIAL APPLICABILITY

A light-emitting device according to the present invention forms a thinfilm of a semiconductor on a flexible material including a plasticsfilm, and can be applied to a wide area of uses including a flexibledisplay unit, an IC card and an ID tag.

The light-emitting device can also be applied to a linearly arrayedlight source, a copying machine, a page printer and an integrated drumcartridge.

This application claims priority from Japanese Patent Application No.2004-326684 filed Nov. 10, 2004, which is hereby incorporated byreference herein.

1. A light-emitting device having a light-emitting element comprisingfirst and second electrodes and a light-emitting layer existing betweenthe first and second electrodes, and a field effect transistor fordriving the light-emitting element, wherein the active layer of thefield effect transistor is formed of an amorphous oxide having anelectronic carrier concentration of less than 10¹⁸/cm³.
 2. Thelight-emitting device according to claim 1, wherein the amorphous oxideincludes at least one of In, Zn and Sn.
 3. The light-emitting deviceaccording to claim 1, wherein the amorphous oxide is any one selectedfrom the group consisting of an oxide containing In, Zn and Sn; an oxidecontaining In and Zn; an oxide containing In and Sn; and an oxidecontaining In.
 4. The light-emitting device according to claim 1,wherein the amorphous oxide includes In, Zn and Ga.
 5. Thelight-emitting device according to claim 1, wherein the light-emittingelement and the field effect transistor are arranged on an opticallytransparent substrate, and a light emitted from the light-emitting layeris output through the substrate.
 6. The light-emitting device accordingto claim 5, wherein the field effect transistor is arranged between thesubstrate and the light-emitting layer.
 7. The light-emitting deviceaccording to claim 1, wherein the light-emitting element and the fieldeffect transistor are arranged on an optically transparent substrate,and a light emitted from the light-emitting layer is output through thesubstrate and the amorphous oxide.
 8. The light-emitting deviceaccording to claim 7, wherein the field effect transistor is arrangedbetween the substrate and the light-emitting layer.
 9. Thelight-emitting device according to claim 1, wherein at least one of thedrain electrode of the field effect transistor and the second electrodeis formed of an optically transparent electroconductive oxide.
 10. Thelight-emitting device according to claim 1, wherein the light-emittingelement is an electroluminescent element.
 11. The light-emitting deviceaccording to claim 1, wherein a plurality of the light-emitting elementsare arranged at least in a single row.
 12. The light-emitting deviceaccording to claim 11, wherein the light-emitting element is arranged soas to be adjacent to the field effect transistor.
 13. Anelectrophotographic device having a photoreceptor, an electrifier forelectrifying the photoreceptor, an exposing light source for exposingthe photoreceptor in order to form a latent image on the photoreceptor,and a developing unit for developing the latent image, wherein theexposing light source has the light-emitting device according to claim11.
 14. A light-emitting device having a light-emitting elementcomprising first and second electrodes and a light-emitting layerexisting between the first and second electrodes, and a field effecttransistor for driving the light-emitting element, wherein wherein anelectron mobility of an active layer of the field effect transistortends to increase with increasing electron carrier concentration.
 15. Alight-emitting device having a light-emitting element comprising firstand second electrodes and a light-emitting layer existing between thefirst and second electrodes, and a field effect transistor for drivingthe light-emitting element, wherein the active layer of the field effecttransistor includes such a transparent amorphous-oxide semiconductor asto be capable of realizing a normally off state.
 16. The light-emittingdevice according to claim 15, wherein the transparent amorphous-oxidesemiconductor has an electronic carrier concentration of less than10¹⁸/cm³, which is sufficiently few in realizing the normally off state.17. An active matrix display device comprising a light-emitting elementcomprising first and second electrodes and a light-emitting layerexisting between the first and second electrodes and a field effecttransistor for driving the light-emitting element, and a picture elementcircuit arranged into a two-dimensional matrix form, wherein the activelayer of the field effect transistor includes such a transparentamorphous-oxide semiconductor as to be capable of realizing a normallyoff state.
 18. The active matrix display device according to claim 17,wherein the transparent amorphous-oxide semiconductor has an electroniccarrier concentration of less than 10¹⁸/cm³, which is sufficiently fewin realizing the normally off state.
 19. A display article comprising; alight-emitting element comprising first and second electrodes and alight-emitting layer existing between the first and second electrodesand a field effect transistor for driving the light-emitting element,wherein an active layer of the field effect transistor includes anamorphous oxide semiconductor.
 20. The display article according toclaim 19, wherein the amorphous oxide is any one selected from the groupconsisting of an oxide containing In, Zn and Sn; an oxide containing Inand Zn; an oxide containing In and Sn; and an oxide containing In. 21.The display article according to claim 19, wherein the transistor is anormally-off type transistor.